By testing painted ordinary cement mortar and composite cement mortar on the dorsal surface of concrete，the effect of ordinary cement mortar and composite cement mortar on concrete impermeability was compared. The influence of composite cement mortar thickness and additive content on impermeability effect was studied.Test results have indicated that composite cement mortar improves concrete impermeability more significantly than ordinary cement mortar.Within a certain range，the thicker the additive content or composite cement mortar，the better the permeability effect. Painting the composite cement mortar on the dorsal surface of concrete is easy to operate in basements and tunnels，which greatly facilitates the construction，so there is a wide range of practical engineering applications.