Abstract: The PC/ABS matrix composites with calcium stearate-modified phosphogypsum (CSMP) were prepared by a melt extrusion method. The effect of contents of calcium stearate and CSMP on the mechanical properties of the composites was investigated, and the modification effect of phosphogypsum was evaluated by means of the contact angle of oil absorption value. Moreover, the microstructure and rheological properties of CSMP were studied by SEM and rheological test. The results indicated that the reaction between calcium stearate and phosphogypsum surface reduced the hydrophilicity of phosphogypsum. An optimum modification effect on mechanical properties was achieved for the composites when 0.5 wt % of calcium stearate was used to modify phosphogypsum. The optimum addition amount of CSMP was determined as 10 wt %. With such an optimum amount, the composites achieved the tensile strength of 45.29 MPa and bending strength 137.87 MPa, which increased by 9.10 % and 25.27 %, respectively, compared to pristine PC/ABS alloy. Moreover, the impact toughness also reached 11.50 kJ/m2, getting an increase by 26.37 %.