In actual conditions,because of the deformation of the box,bearings,shafts and gears in a helicopter reducer,the existence of installation error,manufacturing error and random error,the influence of temperature and vibration,the spiral bevel gear produces misalignment,and they destroyed the ideal meshing condition. The misalignment causes the contact pattern to deviate from the ideal position,at the same time,the transmission error curve also changes,which result in the irregular running,vibration,shock of gear set.Therefore,according to the deviation of the contact pattern to identify the equivalent misalignment,feed it into the tooth surface design,this technology has important engineering significance for improving the meshing quality of gear pair under actual conditions.