The invention provides a water pump with long service life and high sealing performance and an assembling method, and relates to the technical field of assembling of the water pump with high sealing performance. The water pump comprises a pump body support, a motor support, a motor, a coupler, a lead screw, a commutator, a piston and a cylinder body. The pump body support and the motor support arein butt joint. The motor is fixedly mounted on the motor support. The coupler is arranged in the motor support. The lead screw and an output shaft of the motor are coaxially arranged. One axial end of the lead screw stretches into the motor support and is in butt joint with the output shaft of the motor through the coupler. The piston is in butt joint with the other axial end of the lead screw through the commutator. The cylinder body sleeves the outer wall of the piston. The motor drives the lead screw to rotate through the coupler. Rotation of the lead screw is converted into axial transverse movement of the piston through the commutator. The problems that movement of a piston part of a water pump with high sealing performance is blocked, radial runout is large, the service life of thewater pump is short, and the sealing leak rate exceeds standards are solved, and the purposes that the assembling process is digitally detected, the problem about the performance of the water pump ischecked in time, the production efficiency is improved, and the development period is shortened are achieved.