This paper formulates the bandwidth allocation problem in train-to-ground wireless communication networks in operational process of trains. It is shown that the Nash Bargaining game provides an Asymmetric Nash Bargaining Solution which is fair to bandwidth allocation problems in different services. We proposed a bandwidth allocation model for train-to-ground communication system. It can effectively reflect allocation strategies for services with different bargaining power. We define a dynamic adaptive function of bargaining power in order to match utility functions under diverse bandwidths. Easily to implement as it is, an algorithm for bandwidth allocation is derived and then simulated. The simulation show that the proposed scheme has high rate of resource utilization, and is suitable for train-to-ground communication systems.