A back-illuminated p-i-n-i-n separate absorption and multiplication(SAM)solar-blind ultravio. let AlGaN avalanche photodiode(APD)with a low-Al-content p-graded AlxGa1-xN layer and a high/low-Al-content AlGaN multiplication layer was designed. Simultaneously, an AlN/Al0. 64Ga0. 36N distributed Bragg reflector(DBR)structure was inserted to improve the solar-blind photo-response of the designed APD. To fabricate a specially designed AlGaN APD under experimental conditions, the rationality of the device structure needs to be verified in advance. Therefore, the designed APD was simulated using Atlas-Silvaco simulation software, and a conventional APD structure was used as a reference. The simulation re. sults indicate that the designed APD exhibits enhanced optoelectronic characteristics in comparison to a conventional APD, which is attributed to the higher hole-initiated impact ionization coefficient and polariza. tion electric field generated in the same direction as the applied bias field of the designed APD. The de. signed APD exhibits a 10-fold avalanche gain, which is 6. 11×104, and reduced avalanche breakdown voltage compared with a conventional APD. Moreover, the designed APD exhibits good optoelectronic characteristics, and willprovideasolidtheoreticalbasisforthefurtherdevelopmentofAlGaN-based APDs.