Abstract: In strong laser host device, various optical lens with different focal length are needed to expand and collimate the beam, the accurate of the lens'focal length directly affect the overall performance of the system, so it is particularly important to accurately measure the focal length of lens. A new method of measuring the infrared optical system with long focal length by using the Hartmann-Shack wave-front detected device and rotated plane mirror was proposed. The measuring principle of the system was given. Using optical simulation software ZEMAX, the influence of different aperture on the measurement of focal length was analyzed. Then the relationship between the rotation angle of the plane mirror and the system measurement error was deducted by using error theory. Finally the experiment was done to obtain the real result, the result of measurement shows that when the plane mirror rotated to a certain angle, set an appropriate optical aperture, the system measurement precision can be better than 3, much higher than the traditional measuring method. In addition, the measurement method is also conveniently to use, high stability in measure, etc.