A separate absorption and multiplication (SAM) 4H-SiC avalanche photodiode(APD) was designed by using the simulation software of ATLAS. The influences of various thicknesses and doping concentrations of epitaxial layers on spectral response were analysed, and the parameters of multiplication layer were optimal simulated. Then the optimal thickness of 0.26 μm and doping concentration of 9.0×1017 cm-3 for multiplication layer were obtained. The simulation results showed that the APD exhibited low breakdown voltage of －66.4 V with high gain of 105. At the bias of 0 V, the peak responsivity was about 0.11 A/W and the corresponding quantum efficiency was 58%. The UV-to-visible rejection ratio of 1.5×103 close to the breakdown voltage and the maximum spectral detectivity about 1.5×1016 cmHz1/2W-1 were also achieved. The above results indicated that the APD had a good performance for UV signal detection.