In order to realize high precision long-distance laser ranging, a laser ranging method based on the techniques of pseudo-random code (PRC) phase modulation and coherent detection is proposed. An experiment system for the free space target detection is set up. When laser outputs power, local oscillator power, modulation rate, PRC sequence length in a single cycle and telescope aperture are respectively 2.0 mW, 117.8 μW, 100 MHz, 81.9 μs and 2 cm, the system can achieve a ranging accuracy of 11.8 cm for the free space target located in about 8 m far away. The experimental results prove the feasibility of the proposed method. The experimental results are back-propelled to the spaceborne conditions and compared with the GLAS (Geoscience Laser Altimeter System) system parameters. The comparison results show that the system operates under lower peak power and longer pulse width conditions, so high-frequency detection can be achieved.