I use Fender cc60-sce semi-acoustc guitar and Vox mini 5 rhythm amplifier .The amp has 3 modes 0.1W,1.5W and 5W.When I switch to 1.5 W or 5W my guitar's B string starts vibrating rapidly and makes a weird noise and it keeps on vibrating until you mute it by your hand or switch back to 0.1 W.Any solution?
Acoustic and semi acoustic guitars are prone to this sort of phenomenon - feedback. It's caused by sympathetic vibration, where an open string 'hears' its own pitch, gets excited by it, and starts to vibrate in sympathy. It ay be the guitar itself, or something in the room, that picks up on that frequency. At that point, the two sounds egg each other on.
Obviously, less volume stops it happening so much in the first instance, so turning down guiar and/or amp will stop it. As Todd says, muting (any) strings not played is always a good move for amplified guitars. Moving closer to the speaker probably makes it worse - so moving away should improve things.
As an experiment, try it in a different room, guitar away from amp/speaker. Also, try tuning to Eb instead, listen to the difference. It may just be that your guitar's body's natural frequency is B.
Angle your amp away from the guitar.
Αs @Tim said, it is feedback from sympathetic vibrations. But that probably doesn’t help you too much. But it is a loop of the sound going from the amp back into the guitar back out of the amp and back into the guitar over and over until you mute the string or prevent the amp from being able to vibrate the string.