With the increase of renewable energy and DC external power supply capacity, it is more and more important to ensure the frequency safety of the power system. A simplified frequency simulation model is proposed, which includes the synchronous generator, governor and load model, and takes the voltage variation affecting the load response into account. The model is used to simulate the frequency response of the power system under the actual and expected faults. It shows that the simulation accuracy meets the requirements while compared with the calculation results of the BPA detailed model. The simplified model is used to evaluate the current and future frequency security situation of East China Grid. It is pointed out that the frequency security of East China Grid is facing severe challenges in 2025. The primary frequency response services of the conventional synchronous generator need to be improved and the proportion of DC external power supply injecting power needs to be limited to ensure the frequency safety of the power system. It is also suggested that the renewable energy should provide virtual synchronous generator control and primary frequency control to realize the friendly connection of new energy generating units to the grid and meet the needs of the grid development in the future.