The closing and switching operation is related to the reliability and economy of power supply. In order to improve the success rate of closing and switching by the regulation and operation personnel, and the closing current of the power grid cannot be calculated and evaluated accurately. Based on the calculation and analysis of equivalent impedance, combined with the voltage vector collection of 5G measurement terminal, the calculation and evaluation method of the loop closing current was proposed, and then the calculation and evaluation method of loop closing current was carried out by taking the actual cross region loop closing point of power grid as an example. The results show that the closing transient current and impulse current in real time could be calculated accurately, which provided a deterministic basis for regulating the closing and switching operation of operators. The strong feasibility and practicability of the method for calculating and evaluating the loop closing current of power grid was verified.