To enhance the avalanche ionization,we designed a new separate absorption and multiplication AlGaN solarblind avalanche photodiode (APD) by using a high/low-Al-content A1GaN heterostructure as the multiplication region instead of the conventional A1GaN homogeneous layer.The calculated results show that the designed APD with Al0.3Ga0.7N/Al0.45Gao.55N heterostructure multiplication region exhibits a 60％ higher gain than the conventional APD and a smaller avalanche breakdown voltage due to the use of the low-Al-content Al0.3Ga0.7N which has about a six times higher hole ionization coefficient than the high-Al-content Al0.45Ga0.55N.Meanwhile,the designed APD still remains a good solar-blind characteristic by introducing a quarter-wave AlGaN/AlN distributed Bragg reflectors structure at the bottom of the device.