The invention relates to a deviation adjusting method for a sewage treatment parallel-rail bridge crane. The deviation adjusting method comprises the steps of: installing the parallel-rail bridge crane on a settling tank, wherein variable-frequency motors are respectively installed on driving wheels on the two sides of the bridge crane; installing coders on the wheels on the two sides of the bridge crane to feed back wheel speed and position signals; establishing a fault type library and designing an expert rule library containing various deviation adjusting strategies aiming at the fault type library; making a comparison with the fault type library for fault diagnosis according to wheel speed and positioning feedback signals; and automatically adjusting the trajectory of the bridge crane according to a diagnosis result by using solutions in the expert rule library. The deviation adjusting method for the sewage treatment parallel-rail bridge crane has the advantages that the operation stability and the operation safety of a bridge crane system are improved, the service life of main parts such as motors and transmission shafts are improved, special personnel is not required to be assigned for inspecting the parallel-rail bridge crane, the human resource is saved and the enterprise profit is improved.