An on-line diagnosis method of transformer hot fault is given based on infrared thermal imaging technology. Using real-time thermal imaging of transformer surface, the temperature distribution field is built. A heat-transfer model is established in accordance with some parameters, such as surface temperature, internal structure parameters, physical characteristics and environmental temperature of the transformer. And then the internal hot fault temperature of transformer is calculated through the model. By comparing it with DGA diagnosis result, the feasibility of the diagnosis method is proved. And then the on-line and non-contact diagnosis of transformer latent hot fault is realized.