An over-current protection circuit for composite transistor devices is provided, connected between an input terminal and a load, and including: a control-terminal voltage-generation module whose output voltage varies with its input voltage when driven by a first voltage, wherein the output voltage of the control-terminal voltage-generation module serves as a control-terminal voltage; a composite transistor device, connected between the control-terminal voltage-generation module and the load, configured to conduct in response to the control-terminal voltage and a second voltage to generate an output current flowing through the load; and an over-current protection module, connected between the composite transistor device and the load, wherein when the output current of the composite transistor device exceeds a preset limit, a clamping voltage is applied to the composite transistor device by the over-current protection module to limit a current flowing through the composite transistor device, thereby limiting the output current of the composite transistor device.