We report the experimental results to continuously tune the coherence length of a semiconductor laser using an optical feedback scheme. The coherence length can be controlled by adjusting the feedback strength when a semiconductor laser operates in a coherence collapse regime. Fine structures of the fringe visibility of the laser output show that the coherence length of the semiconductor laser can be shortened from several meters of the solitary laser to 100 microm by the long-cavity optical feedback technique. Experimental results indicate that the coherence length of the laser, depending strongly on the feedback strength, is insensitive to its bias current.