A pump with variant strokes has a body, a piston assembly and a decelerating member. The body and the decelerating member are respectively mounted adjacent to the piston assembly. A holder is mounted in the piston assembly and has multiple openings. When the decelerating member is driven to rotate and engages one of the openings in the holder, the piston assembly can move back and forth. Adjusting the decelerating member to connect to different one of the openings of the holder to form different rotating diameters can adjust the strokes of the present invention to provide different pressure and flow capacity.