The self-proposed method to characterize the performance of underwater 3D printing building mortar （3DPBM） was used to study the influence of polyacrylamide（PAM）， cellulose ether （HM） and self-made composite anti-dispersing agent（UAD） on the workability， anti-dispersion performance and mechanical properties of underwater 3DPBM. The results show that when PAM is used as an anti-dispersant agent to prepare underwater 3DPBM， it is easy to make the mortar agglomerate， with poor continuity， difficult to pump out， and unable to meet the requirements of printing on land or underwater. HM and UAD as anti-dispersants enhance the underwater anti-dispersion performance of printing building mortar， which can be used for underwater printing. Moreover， the underwater stacking performance of 3DPBM mixed with UAD is significantly better than that of 3DPBM mixed with HM. Adding a proper amount of PAM，HM and UAD can improve the water-land strength ratio of printing mortar， but if the amount of HM is too large， the water-land strength ratio of printing mortar will decrease instead.