Finally I bought 3.1 roms for my A2000 but when I install it refuses to boot.When I replace it with original ROM(2.04) it starts normally.
Where did you buy the ROM from? A reputable seller or some fly-by-night on ebay? It's possible that you got a defective one, or damaged in transit, or by static electricity during handling. Try again, clean & inspect all pins, make sure nothing obvious (like putting it in backwards, etc.), test it on another board, if you have one. Inspect the ROM socket for corrosion (especially if you've had any battery leaks).
What exactly happens with the ROM installed (when you say "refuses to boot")?
If all of the above fail, contact the seller for a replacement.