I am playing a piano accordion. Seems the bass is very/too loud than the treble even I only pressed the basses quickly and lightly. I googled and saw someone said that Open the box and put a tape across the holes or reeds on the bass side... Is it a proper way to reduce the bass volume, Or have other ways to do it?
You don't mention at all what kind of accordion you play and what registers there are in treble and bass. You also don't mention whether your concern is with the actual bass notes or with the chords on the bass side.
Different instruments have different disposition and registration possibilities and accordingly different ways of addressing imbalances. An instrument designed to be useful for Folk music tends to have a comparatively strong bass side exactly because the normal manner of play would combine a comparatively full registration on the treble side with poignant staccato play on the left.
How much leeway there is for held notes on the bass side and/or comparatively light registration on the treble side depends on the size, design and purpose of an instrument. For some, this can become tricky.