The invention discloses an unlimited stroke piston mechanism with tangent separation, synchronous scrolling and an avoiding effect of cylinders. The unlimited stroke piston mechanism comprises a cylinder body, a left end cover, a right end cover, at least one driving cylinder, at least one driven cylinder, a driving shaft, a driving gear and other structures. A piston of the unlimited stroke piston mechanism does 360-degree rotating movement in the cylinder body; and the other cylinder which is provided with a cycloid avoiding slot and is enveloped by the cylinder body synchronously rotates at a tangent position under the driving of a gear, so that the effects of the input end and the output end of separation fluid for only allowing the piston to pass through and not allowing fluid to pass through in an annular cylinder are realized, and fluid driving of one-way unlimited stroke of the piston is realized. The invention aims at eliminating inherent defects of a traditional piston mechanism and retaining the advantages of the traditional piston mechanism; and the unlimited stroke piston mechanism can be applied to most places where the traditional piston mechanism can be applied, achieves the aims of simplifying the structure, saving energy, being efficient, increasing the power to weight ratio, and the like, and can be widely applied to an unlimited stroke piston type steam engine, an unlimited stroke piston steam catapult, an unlimited stroke piston multistage full force compressor, an unlimited stroke piston automatic transmission and other mechanisms.