Global satellite navigation system is a huge and complex system. If the test verification and ground operators’ training are carried out directly in the real environment, there may be a huge risk in addition to the huge system resources. By establishing the high-fidelity model of space segment, ground segment and user segment of the satellite navigation system, developing the system-level function and performance evaluation algorithm, and realizing the system indirect port simulation defined by ICD file, developing a global satellite navigation system level simulation and analysis software to support “BeiDou III” -- BDSim. This paper focuses on BDSim as a system-level test simulation platform for system testing and evaluation and ground operator training. Firstly, the method of constructing satellite navigation high fidelity model system is proposed, and the model system of BDSim is given. Then, according to the characteristics of “BeiDou III” step-by-step implementation and stepwise networking, the system level test and evaluation method of integrating heaven and earth with "virtual and real integration" is proposed. Finally, according to the characteristics of high complexity and high accuracy of “BeiDou III” ground segment, an immersion operators’ training and assessment method based on BDSim high-fidelity model system and system real data drive is proposed. All relevant methods have been effectively applied in engineering practices. The results show that the methods proposed in this paper can provide technical support for the joint debugging test of each subsystem of the satellite navigation system, the functional and performance test of the system level, and the training and assessment of ground operators, which is of reference significance for the construction of “BeiDou III” related projects.