A novel long-wavelength photodetector with flat-top steep-edge response has been fabricated,which can be used for wavelength division multiplexing(WDM) system.By using low pressure metal organic chemical vapor deposition(LP-MOCVD),the step-shaped GaAs/AlGaAs distributed Bragg reflector(DBR) and the InP PIN photodetector are grown on GaAs substrate,and by employing a thin low-temperature buffer layer,the high quality GaAs/InP heteroepitaxy is realized.The structure of the photodetector is optimized by theoretical simulation.This device has good performance in the flat-top passband:the 0.5 dB bandwidth is 3.9 nm and the 3 dB bandwidth is 4.2 nm.The peak quantum efficiency of 25% around 1 549 nm and the 3 dB bandwidth of 17 GHz are simultaneously obtained.