At present, the evaluation of the mental workload of pilots has become the focus of the development of the aerospace industry. Aiming at the defects and disadvantages of off-line and subjective mental workload evaluation methods. The goal of this study is to evaluate the mental workload of pilots online. The method is based on a Cognitive Task Load (CTL) model, combined with FlightGear flight simulator and Skill-Rule-Knowledge framework. The real flight mission of the aircraft is simulated, and the mental workload of the pilot at the current moment is estimated in real time by calculating the process of the pilot's mental workload during the period from the taxiing phase to the beginning of the cruise. The results show that the method of evaluating the mental workload of pilots by CTL model is effective and reliable, overcomes the limitations of traditional methods, and has strong maneuverability.