For increasing the data rate and reducing the power consumption in satellite communication systems, free-space optical communication technology has been verified as an attractive alternative to traditional radiofrequency communication systems. In this study, a high-sensitivity homodyne coherent optical receiver was demonstrated. A sensitivity of -59.2 dBm was achieved with an unencoded bit-error rate below 10-3 at a bit rate of 1 Gbps (9.4 photons/bit does not include any overhead for forward error correction). To achieve homodyne detection, a modified decision-driven Costas optical phase-locked loop and a digital control algorithm were used to suppress the phase error and track the Doppler shift. Test experiments show that the receiver sensitivity penalty caused by the phase error is approximately 1 dB and that the receiver can track a Doppler shift at the speed of 100 MHz/s. Test experiments demonstrate that the receiver has a robust adaptive capacity to received-power fluctuation. The theoretical analysis and experimental results are presented in this paper.