The invention discloses a telescopic crank device for an engine crankshaft, which relates to the engine field. The device is composed of a lower bracket 1 of a movable crank, a spring loop bar 2, a reset spring 3, the movable crank 4, a crank sliding sleeve 5, a crankshaft output shaft 6, a buffer spring 7, an upper bracket 8 of the movable crank and a crank connecting rod shaft 10. The telescopic crank device suitable for a single-cylinder, double-cylinder, three-cylinder or four-cylinder or multi-cylinder internal combustion engine and compressed-air engine is simple in structure, can effectively utilize a large moment generated under work and improve the energy utilization efficiency.