The uncertainty of new energy power generation poses a great challenge to the transmission system. Since the transmission cable has a certain overload capacity, it can be overloaded for a short time under the premise that the temperature does not exceed the limit value. A method of making full use of the overload level of the cable is proposed to improve the transmission capacity of the cable to new energy power generation in a short time. This method firstly uses the finite element method to calculate the cable temperature field distribution, establishes the allowable overload running time of the line and analyzes its influencing factors. Then the improved BP neural network algorithm is introduced to predict the overload time in combination with the actual power generation output curve. The results show that the BP neural network model has high accuracy and can be applied to evaluate the cable limit transmission capacity and provide rapid support for dispatching decisions.