The invention discloses a variable compression piston. The variable compression piston is characterized in that a cambered surface of the variable compression piston and a cambered surface of a combustion chamber of a steam cylinder head are arc, the cambered surface of the variable compression piston constitutes a second combustion chamber, and the second combustion chamber and the combustion chamber of the steam cylinder head jointly conduct work; at least one annular groove and a telescopic rod which is protruded out of the inner face are arranged at the periphery of the variable compression piston, an external thread inner circle through hole is designed in the outer circle of the telescopic rod and is provided with a sparking plug base, transfer holes which are communicated with an air inlet side and an air outlet side are formed in at least two sides of the cambered surface of the second combustion chamber, and the two sides refer to the position, opposite to the second combustion chamber, of the air inlet door and the air outlet door. The sparking plug base moves synchronously with the variable compression piston, and it is guaranteed that flames of a sparking plug is maintained in the center point of the second combustion chamber all the time, which is beneficial to ignition and spreading of combustible mixture gas.