Based on the theory of energy minimization, a numerical algorithm was developed to calculate friction force distribution along the time-varying contact line in helical gear system. The friction force distribution varies with the instantaneous position of the meshing point and the length of contact line. The friction force was calculated on every meshing point of time-varying contact line via the algorithm. The results show the friction force becomes larger from the tooth root to the pitch point and becomes smaller from the pitch point to the tooth tip. Due to this, there is a significant shock at the pitch point which will generate noise and vibration. The changing law of friction force distribution provides a fundamental theory for modification shape design and impact reduction.