In order to measure the focal length of long focal length laser optical system, a method based on the principle of autocollimation, using fiber laser, beam splitter, flat mirror, precision angular instrument, charge couple device (CCD) and wavefront sensor is proposed. The fiber pinhole is placed at the focal point of laser optical system by testing the transmission wavefront of laser optical system with the Shack-Hartmann wavefront. Then, the CCD is placed at the best image surface by the scanning focus algorithm. The synchronization acquisition of the angular instrument and CCD is made. The focal length of laser optical system is calculated with the angle got by precision angular instrument and the deviation of pinhole image calculated by the centroid algorithm. The long focal length laser optical system of which the focal length is 7171 mm has been tested by this method. The uncertainty of test result is 13.48 mm (k=2). The test results show that this method is practical to the focal length testing of long focal length laser optical system.