Due to low early strength and high shrinkage, ordinary Portland cement (OPC) has difficulty meeting the actual needs of modern construction projects, while belite calcium sulfoaluminate cement (BCSA–OPC) composite cement provides a new solution. The mechanical and the drying shrinkage properties of the BCSA–OPC mortar were determined, the hydration heat of the BCSA–OPC was studied, and the pore size distribution of the mortar was investigated. In addition, the hydration products of the BCSA–OPC were analyzed by X-ray diffraction (XRD) and simultaneous thermal analysis (TG-DSC), and the microscopic morphology of the BCSA–OPC mortar was observed by scanning electron microscopy (SEM). The results show that with the increase in BCSA dosage in the BCSA–OPC, compared with OPC, the flexural strengths of the mortar of 50% dosage of BCSA at the hydration age of 1 d, 3 d, 7 d, and 28 d are improved by 33.3%, 36.6%, 23.6%, and 26.8%, and the compressive strengths are improved by 50.8%, 35.7%, 13.4%, and 27.7%. The drying shrinkage and total porosity of the mortar at the hydration age of 28 d are reduced by 117.4% and 21.55%, respectively. It is attributed to the filling effect of a large amount of ettringite (AFt) and intertwined with the fibrous C-S-H gel to form a network. This study will provide a theoretical basis for the application of the BCSA–OPC engineering.