I'm beginning to learn sight-singing by myself. I get a very low voice, and thus I think I'm NOT ABLE to reproduce those high-pitch notes. Then how is it possible for me to practise sight-singing, if I can't even sing the high-pitch note out?
If you have a bass voice, then you can most easily sight-sing bass vocal music.
However, if you want to sing music with notes in a higher range, this easiest way to do this, would be to sing everything an octave lower (or the sections that are out of your range). Although you will not be reproducing exactly the notes written, in their written octave, this is still a very useful exercise, for two main reasons: it will familiarise you with reading notes in the treble clef; the intervals between the notes remain the same, even when sung in a different octave.
In fact, being able to easily "transpose" by an octave is a very useful musical skill (transpose is in quotes here as the pitch names remain the same). As a guitarist and bass player I am regularly transposing piano music up or down an octave, or between clefs. For a singer it is very useful to be able to sing a different musical part transposed by an octave (for instance a bass voice singing the soprano part), both to be able to help other singers learn their parts, and to help you understand what is happening in other vocal parts.
You could also sing the notes above your vocal range falsetto (using your head voice), although this is not as strong and can feel like quite hard work after some time.
I'd take it further than Bob's as always good answer. Sight singing does not have to rely on being in the written key. If one has absolute or perfect pitch, it may complicate the issue in my answer, but read on. Unlike most instruments, the voice has no particular 'fingering' that's needed to sing, say, a C note. It's more of a relative note thing - C-F is a perfect 4th, but then so is Bb-Eb. Give yourself a sensible start note, and read the intervals from that. If you end up singing a tone down, a fifth up or whatever, you are still practising sight-singing. That way, you won't have to worry what key a piece is in: you can sing any song that is within your tessitura - most songs will be, perhaps with a change of key. When you've practised enough, and become really good, then would be the time to consider singing in the written key for a certain song, but right now, it's academic.
You are asking about practising, so if anyone else would be singing with you, they could do the same.However, you did say 'by yourself'. If someone was sight-singing a new piece, which started on, say, C, but was given ,say, a D instead, the song would still be sung, a capella, correctly, albeit in a different key. Is that such a big deal ? This is all based on your premise that you may not be able to reach certain high notes.
A further thought - it wouldn't matter too much, while learning to sight-sing, if you used treble clef music as well. The intervals are just the same, so once a start note is established, off you go.
Well, falsetto is not head voice (contrary to what the accepted answer writes) and it should certainly be not strenuous before getting quite higher than your natural range. However, it's like switching gears so you cannot really usefully employ it musically since it breaks character. Head voice or mixed voice is a manner of putting falsetto character into the chest voice so you can mask the switch and eventually train it to become a transition. It's still switching gears though, but sort of while letting the clutch slide.
Mixed voice, however, saves the listener more strain than the singer. I've heard falsetto employed by conductors for "marking" notes not properly in their range while demonstrating something.
With regard to actual singing: if you are self-accompanying, electronic keyboards these days all can transpose by intervals different than octaves on the fly. And/or learn chromatic button accordion (there are electronic versions of those as well, and they are nicely compact compared to other electronic keyboards): transposing is pretty simple on those, and transposing by multiples of a minor third is actually trivial.
Or get an accompanist or band that can transpose on the fly.
However, as you work with your voice, you are likely to extend the range you can usefully employ. While you will probably not change the overall range at which you are able to make noises of any kind, much of that range can be put to reliable and amicable musical use eventually.