The invention relates to a transformer winding looseness fault detection method based on an average displacement method, and belongs to the technical field of power transformer safety monitoring. The method comprises the steps that surface vibration data of a transformer with an unloosened winding are measured at preset time intervals within preset time range, the average displacement of the transformer is calculated, and a first average displacement sequence is obtained; surface vibration data of a transformer to be measured are obtained at the same time intervals within the same time range, the average displacement of the transformer is calculated, and a second average displacement sequence is obtained; difference values between the average displacements at the same time delay are calculated, the difference values are added, and evaluation factors for evaluating the winding looseness degree are obtained. The transformer winding looseness fault detection method is easy to implement, easy and convenient to operate and accurate in diagnosis, and the number of needed devices is small.