Using styrene maleic anhydride copolymer (SMA) grafted onto silica nanoparticles, polycarbonate/acrylonitrile butadiene styrene copolymer (PC/ABS) blends were prepared by a two-step method, and the effect of SMA copolymer on the coefficient of thermal expansion (CTE) and impact strength of PC/ABS blends was investigated. The results showed that the addition of SMA copolymer resulted in a decrease in the flowability of PC/ABS blends, while their heat resistance was enhanced. Meanwhile, with the increase of SMA content, the CTE of PC/ABS blends decreased to varying degrees compared to S0, among which the CTE of S1, S2, S3 and S4 respectively decreased by 19.8%, 44.5%, 43.1%, and 41.4% compared to S0. The addition of SMA copolymer effectively improved the LOI value of PC/ABS composites, with S2 sample having the highest LOI value of 28.8%, and the UL-94 measurement result was V-0, with no dripping observed. In addition, with the increase of SMA copolymer content, the tensile strength of PC/ABS blends shows a trend of first increasing and then slightly decreasing, the bending strength shows a trend of first increasing and then stabilizing, the elongation at break shows a trend of first decreasing and then stabilizing, and the notch impact strength shows a continuous upward trend. Among them, the S2 sample has the highest tensile strength, bending strength, and high notch impact strength, with values of 41.6 MPa, 102.4 MPa, and 15.3 J/m², respectively, which are 29.6%, 10.7%, and 1.35 times higher than the corresponding parameters of the S0 sample.