Isometric modification, as a new modification method of spur bevel gear was proposed. Based on the gear geometry theory and the normal meshing motion equation of gear pairs, changes of meshing points and angles are analyzed and the effect of axial modification on gear pair meshing movement is discussed, and then dynamics emulation analysis was operated with FEA software, the results show that the stress distribution of gear surface was controlled by isometric modification, the load concentration, agglutination and pitting of the gear can be avoided effectively.