The invention relates to a press fitting mechanism. The press fitting mechanism comprises a vertical plate arranged on a linear movement module, a press fitting rack arranged on the vertical plate in a sliding manner, a press fitting plate arranged at the bottom end of the press fitting rack, a secondary press fitting table arranged at the bottom end of the right side of the press fitting plate, a primary press fitting air cylinder arranged on the vertical plate, a primary press fitting plate which is connected to an air cylinder shaft of the primary press fitting air cylinder and arranged on the press fitting rack in a sliding manner and a primary press fitting rod which is arranged at the lower end of the primary press fitting plate and penetrates a waterproof jacket grabbing table arranged at the lower end of the secondary press fitting table. According to the press fitting mechanism and a press fitting method thereof, press fitting of a water valve waterproof jacket is achieved in a two-time press fitting manner, the influence of elasticity of the water valve waterproof jacket on assembly is eliminated, the assembly precision of the water valve waterproof jacket is improved, and the percent of pass of products is guaranteed.