The invention discloses a power supply for function debugging of a UPFC converter valve and a controlling method thereof. The power supply comprises a transformer; a primary side of the transformer is connected in series with a current-limiting resistor and a main circuit breaker and then is connected with a power source unit; and by-pass switches are connected in parallel to the two ends of the current-limiting resistor. Grounding switches are connected in series between connection nodes of the current-limiting resistor and the main circuit breaker. A plurality of secondary sides are arranged at the transformer, wherein the number of the secondary sides is not less than the number of to-be-debugged UPFC converter valves; a single-phase rectification bridge is connected between any two phases of each secondary side; and an output terminal of the single-phase rectification bridge is connected in parallel with a filter capacitor. In addition, the invention also discloses a controlling method of a power supply for function debugging of a UPFC converter valve. Before usage, the main circuit breaker is turned off and the by-pass switches are in an off state; first-stage charging is carried out on the filter capacitor; when the output terminal voltage of the single-phase rectification bridge reaches over 400V, the by-pass switches are turned off; and second-stage charging is carried out on the filter capacitor. With the power supply, a phenomenon that the converter valve is damaged due to an impact current can be avoided.