Available Transfer Capability (ATC) is a measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses. Putting forward against the defect that the traditional contingency selection method uses single index to enumerate states or disturbances, this paper introduces the active power behavior index and the voltage-reactive power behavior index, and uses the entropy weight to rank and select the states which seriously affect the system; then from the point of both safety and economy, optimal power flow model is used in every power flow calculation; last introduces the maximum entropy principle to research the probability distribution of ATC. By simulating on the IEEE-14 bus test system, the proposed method has the advantages over the conventional approach in contingency selection, calculation accuracy and processing speed.