The present disclosure relates to a pumping assembly, a piston pump and a water flosser. The pumping assembly includes: a cylinder, at least two chambers with different inner diameters being provided in the cylinder; a piston mechanism successively extending through the at least two chambers, the piston mechanism being in slidable cooperation with walls of the chambers, and a gap being formed between a sidewall of part of the piston mechanism and the walls of the chambers; and a drive mechanism connected to the piston mechanism, the drive mechanism being configured to drive the piston mechanism to move.