The invention provides a device for detecting internal electrical performance of a converter valve sub-module and a control method. The device comprises a transformer, a transformer isolation switch,test load, a load isolation switch, a test load, a load isolation switch, an oscilloscope and a controller, wherein the controller is used for controlling an output to charge a sub-module capacitor through a circuit, consisting of a positive electrode or negative electrode of a power supply, a -tested valve section, a valve tower or a diode and a capacitor of a valve hall bridge arm when the off state of the transformer isolation switch and the on state of the load isolation switch are controlled and after an alternating current power supply is boosted through the transformer, and start voltage test and voltage withstanding test of a sub-module power gaining energy supply are sequentially completed; when the on state of the transformer isolation switch and the off state of the load isolation switch are controlled, the electric quantity of the sub-module capacitor discharges to the test load through the circuit, and sub-module IGBT on-off test, by-pass switch protection action functiontest and power-gaining power supply locking test are sequentially completed. The device and the control method, adopting the scheme, improve efficiency and reliability of the internal electrical performance of the converter valve sub-module.