The impermeability and other performance of sulphoaluminate cement were studied, which mixed with redispersible polymer powders. With the help of SEM and mercury intrusion apparatus, the morphology of the hydrates and the pore structure of the cement mortars were observed. The impermeability mechanism of redispersible polymer powders in sulphoaluminate cement mortar was analyzed. The results show that sulphoaluminate cement mortar could be modified by adding redispersible polymer powders. The flexural strength of sulphoaluminate cement mortar could be increased by adding redispersible polymer powders, and compressive strength of sulphoaluminate cement mortar could also be increased to a certain extent. When the mass fraction of redispersible polymer powders was 0.9%, the flexural strength and compressive strength reached 9.2 MPa and 52.5 MPa. When small amount of redispersible polymer powders was added, the impermeability of modified sulphoaluminate cement mortar was improved significantly. When the mass fraction of redispersible polymer powders exceeded 0.9%, impermeability of Sulphoaluminate cement mortar would not be improved significantly. Due to redispersible powders gap filling and film forming, the interface of cement and aggregate is more closed, total porosity decreased and unharmful porosity increased when redispersible polymer powders is added.