Abstract: Advanced multi-level modulation format has shown its great potential in high-speed and high-spectral-efficiency optical communications at sea. The quadrature phase-shift keying (QPSK) modulation format was introduced into the free-space optical (FSO) communications. Experiments demonstrate that the high-speed FSO transmission links of up to 50 Gbit/s can be achieved, and the eye diagrams and constellations of the transmitted QPSK signals can be clearly observed. By comparing the bit error rate (BER) curves before and after transmission, it is found that the receiving sensitivities are both less than -36.7 dBm for the transmissions of the FSO link, and their power penalties are both less than 4.2 dB at the BER of 3.8×10^-3.