Abstract: In order to improve the accuracy of focal length measurement for long-focus focal infrared optical system, a new method is proposed, that includes interferometry, photoelectric auto-collimation, and laser tracking technique. Based on the wavefront interferometry optical path, it can be realized that the focal length for long-focus infrared optical system is measured with high accuracy, by using the features that the power of wavefront is very sensitive to the position of spherical mirror, and combining with the high-precision position measurement function by laser tracker and the high-precision angle measurement function by photoelectric autocollimator.A long-focus infrared optical system for aerial reconnaissance was tested in the experiment, whose theoretical focal length was 1 520 mm, and the testing error was analyzed according to the accuracy of the equipments. The standard deviation of 5 measurments results was 0.930 mm, and the measurement error was less than 0.2%, meeting the test technical requirements. The experimental result shows that the method is successful and effective to measure the focal length of long-focus infrared optical system accurately.