The service frequency is an important factor which shows the supply and service level of public transportion. By considering the effect of occupancy rate on generalised cost firstly, the authors establish a new public transport model based on generalised cost. Then from the view of optimal pricing, this paper constructs two optimal service frequency models by the aim of maximizing social welfare and maximizing profit separately. The result shows that, without the effect of externalities, the two optimal service frequency is the same. Finally, this paper use some parameters of generalised cost, which coming from a city, to simulate this model, and the result shows it is feasible.