Considering the fact that distribution transformer winding will be deformed on the impact of the short-circuit current, this paper proposes a novel on-line diagnosis of winding deformation method for distribution transformers.Transformer winding deformation leads to changes in the transformer short -circuit reactance，so we can diagnose the extent of deformation of the distribution transformers by measuring the amount of change of the short-circuit reactance. First of all, the relation expression of the short-circuit reactance with respect to the voltage and current of the primary and secondary side is developed based on the equivalent circuit model of the transformer. And then the short-circuit reactance and the degree of winding deformation are computed by using numerical methods and the value of voltage and current of the transformer. The test results indicate that the proposed method is effective and accurate.