The research work is aiming at optimization design and decreasing flux leakage. Using magnetic methods, flux leakage model was established on the model of permanent magnet retarder, which deduced equations on design parameters. Considering the complex function relationships among flux leakage factors, interval mathematics was used to optimize design for permanent magnet retarder. Based on Tschernikow algorithm, nonlinear equations of magnetic flux leakage were simplified into linear equations. In according with design, value intervals were selected, which were rotor flux densities Bm, air gap flux densities Bg,ave and flux leakage coefficient K. With interval iterative methods, intervals and proportions pi on parameters were obtained. Design parameters were searched with Matlab, which reduces amount of calculation and cycle index. 3-D finite element model on flux leakage and test showed the rationality of design parameters and interval mathematics. The results showed that permanent magnet high HPM and air gap ge influence magnetic flux leakage coefficient K, and the validity of Tschernikow algorithm.