Optical phase lock loop (OPLL) is an important part in the coherent laser communication system. A castas digital optical phase lock loop system is designed, which is used in the inter- orbit coherent laser communication and Doppler frequency shift tracking. The system uses field programmable gate array (FPGA) as the central processor. The local laser (LO) is 1550 nm central wavelength sideband modulation injection locking laser, which has high tuning speed and narrow linewidth. Compared with the traditional OPLL, the proposed system provides more stability and frequency acquisition speed. It can achieve phase lock in frequency deviation range of 140 MHz and the phase locked time is longer than 1 h in the experiment. It can also achieve 20 MHz/s Doppler frequency shift tracking and provide the experimental supports for followed- up inter- orbit coherent laser communications.