An algorithm for calculating available transfer capability（ATC） of power system in power market is presented. An optimal mathematical model considering the bidding of generators and the bus locational marginal price（LMP） is built. The objective function is to maximize the total active power consuming for load in sink area. The constraint is the system operation in a secure and economic way. A bi-level leader-follower decision-making measure is taken to solve the problem and semi-smooth Newton method is used to handle the large amount of inequality constraints which are imported by sub-optimal model. The numerical simulation results on IEEE 30-bus system demonstrate the validity of the proposed model.