Power line protection where the current circuit is modified cannot be put into operation until the on-load test has been carried out. The problems of a normal on-load test are that it is inefficient, unstable and of high risk when modifying the circuit loop. Thus this paper proposes a new on-load test based on capacity current when the power line is unloaded. It demonstrates the characteristics of no-load charging current and explores the application range of this new method. The safety of modifying the circuit loop of this method is proved theoretically and compared with the normal on-load method. This new on-load test method is proved to be effective in practice in a 220 kV power station. The result shows that the on-load test based on capacity current will help simplify operational procedure with a precondition of ensuring accuracy, and this new method will dramatically promote the production efficiency of power equipment. This work is supported by the National Natural Science Foundation of China (No. 51877017).