The utility model provides a reset spring position-restricted device of a brake cylinder and an assembly method thereof. The cup-shaped reset spring position-restricted device is slidably disposed in a cup-shaped piston, and a spring is disposed therein in a packaged manner. The side wall of the reset spring position-restricted device is provided with a plurality of grooves and a plurality of slots used for limiting a plurality of finger-shaped parts are disposed in the side wall of the reset spring position-restricted device. Flanges are protruded outwardly on the end parts of the finger-shaped parts in a radial direction, and a position-restricted ring can be disposed of the locked position in the piston in a locked manner. By adopting the reset spring position-restricted device and the piston, the finger-shaped parts outside the piston can be compressed in the inserting position along with the reset spring position-restricted device, and then the flanges can be tightly engaged with the spring. When the compressed reset spring position-restricted device is inserted in the plug, the radial compression of the finger-shaped part can be released, and then the finger-shaped parts can be opened to the locked position from the locked position, and then the deviation of the finger-shaped parts can be stopped.