Abstract: In the application of coherent space optical communication, traditional lasers have the problems of wide linewidth and severe intensity noise, which tends to lead to losing lock of phase locking loop. Singl-frequency Nd:YAG no-planar ring oscillators (NPROs) were developed, with linewidth smaller than 1 kHz and relative intensity noise (RIN) lower than -150 dB/Hz. An optical phase locking loop was constructed and phase locking of two NPRO lasers was realized with -67 dBm signal laser power. The analog communication experiment was conducted at the signal frequency is 10 MHz and 1.25 GHz. When the signal light power is -60 dBm or -53 dBm, respectively, relative ideal eye diagram could be observed. In the digital communication experiment, the receiver sensitivity is -50 dBm at 2.5 Gbps transmission rate, the bit error (BER) is 3.210-6. System sensitivity of quantum limit could be approached, which is much better than that in traditional IM/DD mode. This performance is suitable for long distance, large capacity space communication.