Abstract: The transfer function of off-line frequency response method is difficult to be applied to the winding deformation detection of transformer in operation. In this paper, transfer functions suitable for online monitoring are constructed and their ability to reflect deformation defects is compared and analyzed. Based on the theory of multi-port circuit network, three transfer functions for the neutral earthed and ungrounded live operating transformers are constructed, respectively. Based on the actual winding parameters of 180 MVA/220 kV transformer, a three-phase double-winding transformer multi-conductor transmission line model is established, and the frequency response curves of each transfer function and eight characteristic parameters are calculated under the conditions of bulging and displacement deformation. The comparison results show that the correlation coefficient, Euclidean distance, linearity with deformation and sensitivity to deformation of the online transfer function are basically consistent with those of the offline transfer function. The linearity of the characteristic parameters of each transfer function is above 0.8, and the sensitivity to bulging deformation is about 40% of the displacement deformation. The constructed transfer functions not only are suitable for live detection, but also have the same performance as the offline transfer function.