The invention relates to a horizontal plastic water cellar and an assembly method of the horizontal plastic water cellar, and belongs to the technical field of water conservancy projects. The horizontal plastic water cellar comprises an end cover, a reinforcing rib I, a radiate rib, an annular rib, a cellar body I, a reinforcing rib II, a flange rib I, a cellar body II, a lock, a well cover, a latch, a shaft, a well cover base, a reinforcing rib III, a reinforcing rib IV, a flange rib II, a cellar body III and a guide hole. The cellar bodies are integrally formed at a time or are formed by rolling and welding a board, and standardized and mass production is facilitated. The water cellar is welded and assembled, site operation is easy, and installation is convenient. The number of the middle cellar bodies can be increased and decreased flexibly to adjust the volume of the water cellar. The cellar bodies are connected through the flange ribs and the reinforcing ribs. The flange ribs can increase the rigidity of cellar body rings; the reinforcing ribs can enhance the intensity of the connection positions and ensure the leakproofness of the connection positions.