Mechanical property and water resistance properties of desulphurization gypsum were improved by phenolic resin using the hot pressing process. X ray diffraction(XRD), thermo gravimetric analyzer(DTA), scanning electron microscopy(SEM), Fourier transform infrared spectrometer(FTIR) and isothermal calorimeter were employed to study microstructure and hydration mechanism of composite. The results show that optimal parameter in hot pressing process for flexural strength is 8MPa,160℃ and 120s, optimal parameter in hot pressing process for compressive strength is 8MPa,180℃ and 75s; when content of phenolic resin, silane coupling agent and expansive agent are 1.0%, 0.3% and 3.0%(by mass) respectively, water absorption(2h) and water absorption(24h) are 0.58% and 1.07% respectively. Phenolic resin play the role to fill pores and enhance dense ability and water resistance properties during the hydration process.