With the development of high-performance computers, the contact finite element analysis (FEA) method has become more and more popular for studying both the static and dynamic behaviors of gear drives. In this paper, a precise tooth profile modification (TPM) approach of the helical gear pairs is presented first. The type and amount of the TPM are accurately determined by the static contact FEA results. Then dynamic contact simulations for the helical gear pairs with and without TPM are, respectively, carried out to evaluate the effect of the presented TPM approach on vibration reduction. No additional assumptions and simplifications are required for the static and dynamic contact analysis models. Vibration comparison experiments are also carried out on an open power flow test rig. Both the simulated and experimental results show that the presented precise TPM of helical gears is effective on vibration reduction around the working load, and the dynamic contact simulation is effective in estimating the effect of the TPM on vibration reduction in the designing stage.