The effect of sodium methyl silicate content on the water resistance of desulfurization gypsum block was studied by means of scanning electron microscopy(SEM) and Fourier transform infrared spectroscopy(FTIR). The results show that the strength of desulfurized gypsum block increases first and then decreases when the content of sodium methyl silicate is 02%14%. The surface contact angle is increased, water absorption is greatly reduced, and softening coefficient increased steadily. When the content of sodium methyl silicate is 10%, the water absorption of desulfurized gypsum block is 7%; the softening coefficient is 085 Compared with the control sample, the flexural and compressive strength values have no significant change. FTIR spectral analysis show that sodium methyl silicate can form hydrophobic film on the surface and pore wall of block, which is the main reason for the decrease of water absorption and the improvement of softening coefficient of block. The addition of methyl silicate will change the crystal morphology and microstructure of dihydrate gypsum, thus affecting the strength of the block.