The flight error of RNP flight program is an important index for real-time evaluation of aircraft flying according to the expected route. Most of the current flight error evaluation algorithms stay in the theoretical research stage and lack of flight data samples for verification. In order to solve the above problems, a real-time evaluation method of flight error based on actual navigation performance is proposed in the paper, simulation experiments are carried out on the open-source flight simulation platform BlueSky. The experiments show that this method can complete the real-time evaluation of flight error and also provides an idea for the verification and implementation of other flight error evaluation algorithms.