An adjustment-free multi-stage prosthesis air cylinder comprises an air cylinder body, a piston, a first check valve and a multi-stage air pressure valve. The inner of the air cylinder body forms an air chamber and a lower air way. One end of the piston is slidably disposed in the air chamber and divides the air chamber into an upper air chamber and a lower air chamber, the other end of the piston extends to the outside； the inner of the piston forms an upper air way which is communicated with the outside and the upper air chamber, while the lower air way is communicated with the lower air chamber. The piston makes a reciprocating movement in the air chamber, and the first check valve is disposed in the piston to optionally connect the upper air way with the lower air way. The multi-stage air pressure valve is disposed in the lower air way. With the arrangement of the multi-stage air pressure valve, the users can easily obtain different walking speeds without changing the air cylinder or setting the operational mode.
