The invention discloses a variable cycle engine. The variable cycle engine includes a combustion cylinder, a cooling cylinder, a combustion cylinder combustion chamber, a combustion air cylinder piston, a cooling cylinder piston, a tail gas exhaust mechanism, a back heating chamber, a crank connecting rod power output shaft, a tail gas exhaust valve, a cooling cylinder combustion chamber, a cooling cylinder gas inlet valve, a cooling cylinder circulation gas valve, a combustion cylinder circulation gas valve, a combustion cylinder gas inlet valve and a heat dissipation chamber; the combustioncylinder is provided with a circulation gas valve to control the gas circulation with the back heating chamber; the cooling cylinder is provided with a circulation gas valve to control the gas circulation with the heat dissipation chamber; the tail gas exhaust mechanism is installed at the place close to the heat dissipation chamber; a tail gas purifying device and a heat regenerator are installedin the back heating chamber; and the engine performs cyclic work in a variable cycle working mode. The engine has five power strokes, a Stirling cycle mode is converted at the fifth and sixth strokesto do expansion work, the residual energy of combustion gas working substances can be fully utilized, and the efficiency of the engine can be enhanced.