In responses to the requirements of low noise, high sensitivity and high bandwidth, a quad photodetector in space laser interferometry for detecting gravity waves is developed. The quad photodetector is a hybrid of quad photodetector chip and readout circuits, while the chip is made by four same InGaAs PIN photodiodes with double depletion regions, so as to lower the photodiode capacitance, narrow the gap between quadrants, and improve the detection sensitivity. The detection module integrated with photodetector chip and low-noise transimpedance amplification readout circuits was simulated by PSPICE. By optimizing the circuit parameters, the gain, bandwidth and noise power density were calculated. Test results show that the integrated detection module has a -3dB-bandwidth of 28.3MHz, and a noise power density of 1.7pW/Hz1/2, and the inconsistency between quadrants is 0.76%. Therefore, the quad photodetector can satisfy the basic requirements of space laser interferometry.