The dependence of light output on the surface treatment of BGO crystals has been tested. The results of experiments and Monte Carlo calculation indicate that for tapered ВGO crystal one of the best ways to improve the uniformity and the energy resolution and to obtain higher light output is roughing the surface which connects PM tube. We also observed that different wrapping method can effect its uniformity and resolution. Our Monte Carlo calculation indicates that the higher one of the "double peaks" is the photoelectron peak of γ rays.