To achieve accurate measurement of focal length of lens and optical system, the measurement theory based on Talbot-Moiré is analyzed. The system includes a collimated light generator, two Ronchi gratings and an image acquisition system. The theoretical issues and deviation are analyzed. The measurement uncertainty also increases with the increase of the focal length under test. When the focal length of lens under test is 500 m, the measurement uncertainty is 5.23%. The optical system whose focal length is 75 272.80 mm is measured, and its measuring relative deviation is 1.09%.