Winding deformation has become an important factor affecting the safe and stable operation of transformers. Parameters such as winding resistance, leakage inductance, and capacitance will change as a result of winding deformation. Therefore, a method of diagnosis of transformer winding deformation based on the above three parameters is proposed in this paper. An equivalent model of the transformer including winding resistance, leakage inductance, and equivalent capacitance is established. Online identification of the winding parameters is realized by the recursive least squares algorithm. Both the winding deformation classification and diagnosis of deformation degree are realized by using the winding parameters identified online. A simulation model of the actual transformer is established for verification and analysis. The results indicate that the method has high precision and overall correct rate. It meets the needs of online monitoring of winding deformation engineering. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.