Abstract: Water purification sludge is a byproduct of water treatment in waterworks, characterized by high moisture content, high organic matter content and poor mechanical dewatering performance. In order to study the solidification effect of various types of cement on water purification sludge, ordinary Portland cement (OPC), sulfoaluminate cement (SAC), slag cement (SPC), and magnesium phosphate cement (MPC) were used to solidify the water purification sludge, respectively. The solidification effect and mechanism of different types of cement on the water purification sludge was investigated by unconfined compressive strength (UCS) test and microscopic observation tests. The results show that OPC, SAC, and SPC have poor solidification effects on the water purification sludge; the solidification effect of MPC is good. The 28 day UCS of OPC, SAC, and SPC with a 40% dosage for solidified water purification sludge is 180 kPa, 247 kPa, and 264 kPa, respectively; the 28 day UCS of solidified water purification sludge with 10%, 20%, 30%, and 40% MPC is 336 kPa, 625 kPa, 886 kPa, and 1584 kPa, respectively. Microscopic observation tests reveal that a three-dimensional cementitious structure with MgO particles as the framework and phosphate hydration products as the cementitious phase is formed inside the MPC solidified water purification sludge, thereby enhancing the skeleton reconstruction effect and particle cementitious effect of water purification sludge, and effectively improving the unconfined compressive strength of the MPC solidified water purification sludge.