Aiming at noise harshness reduction during the process of gearing in automobile transmission,from the view of gear geometry,a method to compute the magnitude of bias of helical gear pair was proposed.Meshing equations of driving surfaces,satisfying the condition that the gear pair is instantaneously in tangent contact,were built based on the principle of gear meshing.The problem of solving the equations was transformed to an optimization problem of guaranteeing the minimum normal distance between meshing flanks and minimum deviation between normal vectors.Then the magnitude of bias was achieved.Meanwhile,a model of the target transmission was built with Masta simulation platform,and the computation results were verified by conducting simulation experiments on the Masta.The simulation experiments indicate that,after the bias modification of gear pair,the transmission error is reduced apparently and the distribution of load becomes more reasonable under the common working conditions.