Abstract: The effects due to the stray field from Lambertson septum magnet are important, including beam high order resonance and unstable effect, etc, The 3D numerical simulations with OPERA3D/TOSCA code and the optimizing processes of design of Lambertson septum magnet were described in this paper. The simulations were performed for different models so that we could decrease the ratio of integrated stray field to integrated main field. After optimization of design, including increasing the thickness of septum to 15 mm and adding a shielding hat with radius of 200 mm and thickness of 10 mm, the ratios of transverse stray field to main field were 0.8‰(in y direction) and 0.36‰(in x direction). The results were satisfactory to the requirement of RCS beam dynamics.