The utility model discloses an automatic sewage sampling tank based on pressure difference, and relates to the technical field of sewage. The automatic sewage sampling tank comprises a tank body, a liquid cavity is formed in the inner bottom wall of the tank body, a guide column is fixedly connected to the middle of the tank body, a liquid inlet is formed in the middle of the guide column, a hydraulic cover sleeving the outer side of the guide column is movably connected to the top of the tank body, and a groove matched with the guide column is formed in the middle of the hydraulic cover. According to the automatic sewage sampling tank based on pressure difference, through cooperated use of the tank and the hydraulic cover, before use, compressed gas is supplemented into the gas tank according to the depth of a to-be-collected sample; when the sampling device is used for sampling, and a set depth is reached, the hydraulic cover moves downwards under the action of water pressure, the groove is communicated with the liquid inlet, sampling is started, and compressed gas with different volumes can be filled when sampling is conducted in liquid with different depths, so that the effectof automatically sampling at a fixed depth is achieved, the sampling efficiency is improved, and the sampling work is effectively guaranteed.