Power flow transfer caused by overloaded tie-lines tripping-off is one of the main reasons leading to cascading failure. However, too strict flow restriction of the tie-lines may inhibit the transmission capacity of the line, resulting in a waste of transmission resources. In order to solve the problem of unnecessary load control caused by the change of the sharp-peak load, it is possible to allow the tie-lines to be overloaded in a short time without affecting the safety of the system. Based on the predicted load curve and the ambient temperature change curve, the load flow-temperature model of the tie-lines is established on account of the dynamic thermal equilibrium process. To solve the model, the BS23 algorithm is proposed. The model can be used to calculate the conductor temperature dynamically and determine the short-time overload capability of the tie-lines to improve the transmission capacity. By way of the simulation of a 16-node system, it shows that the proposed method can retain the peak load and keep the line temperature within the allowable range. As a result, the transmission capability of the section is improved effectively.