In this paper, the standard consistency water consumption, setting time, stability and the change law of compressive and flexural strength of sulfoaluminate based composite cementitious materials with different boric acid amounts were studied, and the hydration mechanism of composite cementitious materials was analyzed by XRD and SEM. The results show that the incorporation of boric acid does not affect the stability of cementitious materials, its standard consistency water consumption increases, and the standard consistency water consumption is inversely proportional to the amount of boric acid. The greater the amount of boric acid is, the more obvious the prolongation of the initial and final setting time is. When the amount of boric acid is 0.20% (mass fraction), the early strength of the test group with a high proportion of sulfoaluminate cement increases, and the late strength can not be inverted shrinkage. Boric acid can make the morphology of AFt stouter. The doping of boric acid can make the dry shrinkage rate decrease and the quality change rate show a decreasing trend.