The invention discloses a method for determining the safety operation area of an H-bridge module and a method for determining the system safety operation area of a cascade multilevel converter. The method for determining the safety operation area of the H-bridge module comprises the following steps: obtaining the characteristic parameters of a switch device, a diode and a capacitor of the H-bridge module; obtaining the safety operation boundary conditions of the switch device; obtaining the module safety operation area of the switch device according to the characteristic parameters of the switch device, the diode and the capacitor and the safety operation boundary conditions of the switch device; obtaining the module safety operation area of the diode; and obtaining the safety operation area of the H-bridge module according to the intersection of the module safety operation area of the switch device and the module safety operation area of the diode. Thus, device characteristics can be accurately described, so that the safety operation range of the device in the converter is accurately determined.