The invention relates to a controllable voltage source type submodule protection method, and the method comprises the steps: setting a protection constant value, carrying out temperature sampling and calculation, and carrying out warning or protection operation through a submodule. The method obtains the thermal stress states of key positions and parts of the submodule through measuring the temperature of the key positions of the submodule and the internal temperature, inverted based on a thermal path model, of a power electronic device. Through the reasonably setting of the protection constant value, the method achieves the protection against an overheating fault of a controllable voltage source type submodule.