The effects of ash and slag from a biomass power plant on the compressive strength, setting time and fluidity of the pastes of Portland cement (P.O) and sulfoaluminate cement (SAC) were studied, and the hydration products and microstructure at the age of 7 days were analyzed via XRD, SEM and other test methods. The results show that the compressive strength of the composite cementitious material decreases, the setting time prolongs and the fluidity increases with the increase in the ash and slag content in the power plant. The microscopic analysis shows that the ash and slag of the biomass power plant can promote the hydration of Portland cement and sulfoaluminate cement paste, and increase the generation of hydration products. The results showed that replacing SAC clinker with 20−30% biomass power plant ash (BPPA) decreased the cement strength, and that an appropriate amount of BPPA (10−15%) could significantly improve the mechanical strength of SAC blended cement. The compressive strength of blended BPPA composite cementitious material in 28 days could reach 60 MPa. This study provided solutions to utilizing the BPPA as a building material admixture to minimize the consumption of energy-intensive cement and to meet the growing needs of the construction industry.