A wavelength selective monolithically integrated photodetector array is fabricated and characterized, which can be used for reconfigurable optical add-drop multiplexers. The integrated device consists of GaAs/AlGaAs Fabry-Pérot resonant cavities and InP-In_0.53Ga_0.47 As-InP PIN photodetectors on the GaAs substrate. In order to achieve multiple channel routing detection, the thickness of GaAs-based resonant cavity is varied by wet etching, and then regrowth of the resonant cavities are accomplished by using the second epitaxy. High-quality heteroepitaxy is realized by employing a thin low-temperature buffer layer. The device is operated at a wavelength around 1500 nm for four channels with 10 nm interval. A spectral linewidth smaller than 0.8 nm, an external quantum efficiency of about 12%, and response rate of 8.2 GHz are simultaneously obtained in the device. The experiment result exhibits good agreement with the calculation.