In order to improve the case that the safety margin is a little higher using GB3480 to determine the gear root stress, a method which combines the GB3480 and FEM to determine the basic value of gear root stress is proposed. Using FEM, the comprehensive gear root stress can be figured out firstly based on the accurate tooth profile model. Then the gear root stress can be determined more accurately according to the key coefficients. Using the proposed method, the basic value of the gear root stress for the gears given in this paper is decreased by 11%. The proposed method can provide the useful guide to lightweight design for the gear transmission system.