I have also found another BenQ model(BenQ MS630ST) similar to BenQ MX631St, but the resolution seems lower. Is it a problem for its working? Thanks all!
Native Resolution‎ SVGA (800 x 600)‎
Aspect Ratio‎ Native 4:3 (5 aspect ratio selectable)‎
Throw Ratio‎ 0.9-1.08 (78″ @ 1.44m)‎
http://www.benq.co.uk/product/projector/ms630st/specifications/