In this paper, a new method combining carrier transport in semiconductors with an RF equivalent circuit was put forward to simulate the frequency response of an avalanche photodiode (APD). The main trade-off between the gain-bandwidth product (GBP) and the dark current was analyzed to optimize the structure of an APD; and a separated absorption, grading, charge, multiplication, charge, transit (SAGCMCT) structure with 120 nm balanced InAlAs multiplication layer was proposed to reduce the dark current and improve the frequency response. The fabricated triple-mesa type back-illuminated InGaAs/InAlAs APD achieved the properties of low dark current of 6.7 nA at 0.9Vb and high GBP over 210 GHz.