In this paper, the measurement method of long focal length in a Fizeau interferometer with combination lenses is stated. According to the principle of geometry optics, two focal length calculation formulae are deduced for the measurement method. The optimization of measurement parameter values is analyzed, and the optimized values of measurement parameters are selected. By analyzing the measurement error of the focal length needed to be measured, the permitted error ranges of measurement parameters are obtained for the optimization of parameters so as to meet the requirement of measurement accuracy of focal length (±0.1%). By means of analysis, it is obtained that the measurement method can meet the requirement (±0.1%) of measurement accuracy of long focal length of high power laser Facility.