Abstract: With the wide application of long focal length optical elements in remote-sensing technology, high-power lasers and astronomical telescope systems, the high-precision measurement of such optical elements' focal length is required. Based on the technique of double-grating interferometry, a new improved algorithm was proposed. This algorithm obtained the focal length calculation formula by the functional relation between the Moiré fringe tilt angle and the focal length of the element to be measured, and optimized the calculation formula by using the ideal state of the Moiré fringe when illuminated by a collimated beam. So the step for determining the Moiré fringe's baseline can be avoided, and the precision of the Moiré fringe tilt angle can be improved. In the experiment, two Ronchi gratings with period of 20 μm were used to build the measuring system. The measurement result of the test lens is 36.690 m, and the repeatability accuracy is up to 0.382‰.