Based on the central composite design (CCD) of response surface method, a 3D printing gypsum powder fluidity optimization experiment was designed, and the effect of different dosages of hydrophobic nano silica and soluble starch on the liquidity of 3D printing gypsum powder was analyzed. Tests regarding printer powder coating, surface topography and flow rate for the optimized 3D printing powder were carried out. The results show that the best scheme to improve the fluidity of gypsum is introducing 1% (mass fraction) hydrophobic nano SiO2 and 3% (mass fraction) soluble starch. Compared with unmodified gypsum, the fluidity of gypsum increases by 38%. The nonlinear regression equation between the fluidity of powder and the content of hydrophobic nano silica and soluble starch was obtained, and the relative error between the best optimized predicted value and the measured value is only 0.31%. Under the best optimized condition, the flow rate of the modified gypsum powder is 3.16 g/s, which flows continuously, and the surface of the powder layer is uniform without obvious defects. This research highly improves the accuracy of gypsum-based powder 3D printing, which is beneficial to the application of gypsum materials in 3D printing.