Digital coherent optical receiver has a wide application prospect in the next generation satellite to ground laser communication.Taking the narrow line width DFB laser as local oscillator,combined with 90° optical mixing and automatic frequency control(AFC) loop,a coherent light communication of 1.25 Gbps is realized.The experimental results indicate that the AFC loop can make the frequency difference between signal light and local light keep within 10 Hz,and the phase difference between I branch and Q branch will keep 90° strictly for a long time.The carrier phase compensation technique based on the FPGA can effectively correct the phase residual errors of AFC loop.The sensitivity of the receiver is superior to 54 photons per bit at error rate of 1×10-6 bit.