The invention discloses a box structure of a sink type washing machine. The box structure comprises a sink body, wherein the sink body comprises four side walls connected sequentially and a bottom plate arranged at the bottoms of the side walls, a washing cavity is formed by space defined by the side walls and the bottom plate, and a washing mechanism is arranged in the washing cavity. The box structure is characterized in that a connecting part between every two adjacent side walls adopts smooth transition in a position close to the bottom plate, and a connecting part between each side wall and the bottom plate adopts smooth transition. The connecting parts between the walls of the washing cavity of the washing machine adopt smooth transition, so that water flow is facilitated, potential energy of the water flow can be converted into kinetic energy flowing toward a draining area, the water return efficiency is increased, and the efficiency of a water pump is increased; the bottom plate is arranged in a downwards bulging spherical shape, accordingly, water in all positions of the bottom plate can flow to a water inlet of the water pump uniformly and stably, the water return efficiency can be further increased, and noise can be reduced.