A steam turbine and a method for internally cooling the same. The steam turbine includes an outer casing and an inner casing; a rotor having a balancing piston, the rotor being rotatably mounted inside the inner casing; and a steam flow channel formed between the inner casing and the rotor. Moving blades fitted with the rotor and stationary blades fitted with the inner casing are alternately arranged to form multiple stages of blade groups, and an interlayer for steam to circulate is formed between the inner casing and the outer casing. The multiple stages of blade groups include a first set blade staging and a second set blade staging; and the top of the balancing piston is provided with a first chamber and a second chamber. A first channel disposed in the inner casing connects the flow passage downstream of the first set blade staging to the first chamber; and a second channel connects the second chamber to the interlayer and connects the interlayer to the flow passage downstream of the second set blade staging.