The invention discloses an automatic desilting and dredging device for a sewer. The automatic desilting and dredging device for the sewer comprises a main machine body, a storage cavity is formed in the right side face of the main machine body, a sliding plate is slidably arranged in the storage cavity, a gathering mechanism capable of gathering sludge is arranged at the bottom of the sliding plate and comprises two sets of fixed blocks in front-rear symmetry, the top faces of the fixed blocks are both fixedly connected with the bottom face of the sliding plate, sliding columns capable of being stored are slidably arranged in the bottom faces of the fixed blocks correspondingly, and sweeping rotating wheels capable of gathering the sludge are arranged at the bottoms of the sliding columnscorrespondingly. Through clever design, the sliding plate and the sweeping rotating wheels can be stored, thus the space occupied by the device is lesser, the occupied area of the device is decreased,meanwhile, the sweeping rotating wheels can gather the sludge towards the middle, collection by a feeding pipe is convenient, the desilting effect is enhanced, and the step of manually clearing the sewer is omitted.