Abstract: In ordinary focal length measurement, the measurement accuracy is usually affected by the longitudinal chromatic aberration because of the different design wavelengths between the optical system and the testing equipment. In order to solve this problem, the variation rule of longitudinal chromatic aberration for optical system was established, and the Conrady formula and apochromatic characteristic formula used to express the functional relationship of longitudinal chromatic aberration in the 400 nm~1 000 nm wavelength were determined. According to the linear relationship characteristic of the optical system between defocus amount of near-focus and position, the focal length position at 5 different wavelengths was measured by using Fizeau interferometer to obtain the longitudinal chromatic aberration curve of the single lens and the double cemented lens. Experimental results show that the longitudinal chromatic aberration function relationship of the monochromatic system complies with the Conrady formula, and the longitudinal chromatic aberration function relationship of the achromatic system matches the achromatic characteristic formula in the 400 nm~1 000 nm wavelength. The research provides new ways and references for the theoretical calculation and accurate detection of focal length.