A steam turbine having a steam supplementing structure and an operating method therefor. The steam turbine includes an outer casing and an inner casing, a rotor having a thrust balancing piston, the rotor being rotatably mounted inside the inner casing; and a steam flow channel formed between the inner casing and the rotor. Impeller blades fitted with the rotor and a plurality of guide blades fitted with the inner casing are alternately arranged to form multiple stages of blade groups. Steam is fed from the steam throughflow downstream of a first designated blade staging in multiple stages of blade groups to a thrust balancing piston chamber disposed between the inner casing and the thrust balancing piston of the rotor. An interlayer for the steam to circulate is formed between the inner casing and the outer casing, the interlayer including a supplemental steam chamber which can receive the steam from a sealed chamber between the rotor and the inner casing. The steam is mixed with supplemental steam fed into the steam supplementing chamber via steam supplementing pipelines. The mixed steam then returns, via the communicating pipe in the inner casing, to the steam throughflow downstream of the second designated blade staging in the flow channel.