Since the focal length of the lens based on traditional measurement has large error, the Quasi-Monte Carlo algorithm is adopted for analysis. The liquid crystal display screen differentiation graphics are used instead of parallel Perot plate light pipe. Then least square fitting is used to calculate the focal length. Adaptive Quasi-Monte Carlo algorithm sampler is adopted for imaging sampling, and according to the brightness difference of each pixel and its neighboring pixels, different number of samples are produced. The sampler has no limit control values, so the Hardy-Krause is used to decide the bounded variation function. By Dovi Lebegue method, the error of the Quasi-Monte Carlo method is decided by the star deviation error of random sequence. Simulation shows that the standard deviation of the focal length measured by LCD screen is smaller than that measured by visual measuring, and that root-mean square error of Quasi-Monte Carlo algorithm is smaller than that of the Monte Carlo algorithm.