The phosphogypsum (PG) was heat-treated in an oven with a constant temperature, and the phosphogypsum-based composite cementitious material (PGCM) was prepared by adding lime calcium powder, normal Portland cement, retarder and water reducing agent. The effects of heat treatment time on the splitting tensile strength of PGCM, the reduction coefficient of axial compressive strength and splitting tensile strength, the failure morphology, and the splitting tensile stress-strain curves were studied. The results show that the splitting tensile strength increases first and then decreases with the extension of heat treatment time. When the heat treatment time is 90 min, the splitting tensile strength reaches the maximum 1.68 MPa. The reduction coefficients of axial compressive strength and splitting tensile strength almost reduce with the extension of heat treatment time. The splitting tensile strength is 42% to 92% of axial compressive strength. The failure morphology of PGCM mainly including central cracking failure, local failure and minor crack failure. The splitting tensile stress-strain curves of PGCM gradually rise first and then decrease linearly. Combined with the test data, the conversion relationship between the axial compressive strength and splitting tensile strength of PGCM, and the equation of splitting tensile stress-strain rising section curves are obtained. The fitting results are in good agreement with test data.