Magnesium phosphate cement-based material has the characteristics of fast hardening and early strength,which is suitable for cement-based powder bed 3D printing technology. In order to solve the problem of poor water resistance of powder bed 3D printing magnesium phosphate cement,the optimization of water resistance of powder bed 3D printing magnesium phosphate cement-based materials was carried out. The influence of M/P molar ratio on water resistance of materials was studied,and M/P molar ratio was optimized. The effect of fly ash incorporation on the printing performance and water resistance of powder bed 3D printing magnesium phosphate cement-based materials was studied,and the mechanism of fly ash improvement on water resistance of printing materials was revealed by combining the microscopic characterization methods. The results show that the compressive strength of powder bed 3D printing magnesium phosphate cement-based materials decreases under water curing condition. With the increase of M/P molar ratio,the water resistance of specimen is improved accordingly. The addition of fly ash can effectively improve the density and printing accuracy of the powder bed,increase the amount of hydration products,make the microstructure more dense,and improve the water resistance of specimen. By adding 20% (mass fration) fly ash,the compressive strength of materials increases by 8900% at most after 28 d of water immersion,which provides strong support for improving the water resistance of powder bed 3D printing magnesium phosphate cement-based materials and expanding the application of cement-based powder 3D printing technology.