Experimental research is performed on a phase noise compensation (PNC) method to improve the signal-to-noise ratio of a frequency modulated continuous wave (FMCW) ladar under long-distance-detection and atmospheric propagation conditions. In the PNC method, a local self-coherent reference is generated where phase noise of the laser source is recorded. Then the phase noise of long-distance-target echo can be compensated by mean of the recorded source phase noise. The laser source of the ladar is chirped modulated from 1550-nm seed laser, whose bandwidth is 10 GHz. In the experiments, the target for test is an optical corner cube retro-reflector that placed over 10 km away from the ladar. The local self-coherent reference, also refer as auxiliary reference, is the heterodyne signal between the chirped laser source and the delay of itself. The delay path is a 5-km soundproofed fiber optic ring. A 2D data processing technique is proposed for ranging with the concatenately generated phase method applied to implement PNC. To verify the proposed technique under long-distance atmospheric propagation conditions, experiments are performed with the distances of the target as 12 and 19.5 km respectively, and the spectral linewidth of the seed laser as 100 Hz and 3 kHz respectively. By comparing the experimental results, the proposed technique is effective in compensating FMCW laser phase noise, even when the target is further and the phase noise of seed laser is worse.