Coherent laser communication has become one of the important technologies of inter satellite high speed data communication because of its high communication rate,complex coding format and extremely high sensitivity.In this paper,a QPSK spatially coherent laser communication system link is designed to give the modulation mapping table from the binary electrical signal to the laser carrier phase at the transmitter and the demodulation mapping table from the laser carrier phase to the binary electrical signal at the receiver are given.The digital simulation of the designed spatial coherent laser communication system link is carried out,and the EVM of constellation is used as the core index to measure the system performance.The impact of the parameters such as the input power at the receiver and the noise figure of the optical amplifier at the receiver on the EVM are simulated and analyzed.The corresponding communication link is built according to the simulation results.When the input power is -45 dBm and -48.5 dBm,the corresponding EVM of constellation is 17.8% and 24.4% respectively,which is in good agreement with the simulation results.