The field winding deformation test of the transformer with oil and gas bushing connected can be processed only by acquiring signals at the bushing tap. The influence of the bushing tap and the gas insulated line section makes the amplitude- frequency response curves different from the factory test and commissioning test. The method of constructing equivalent model of transformer is adopted, and the amplitude-frequency response curves of the field test at the bushing tap signal and the commissioning test at the winding end signal are compared. Differences and production principles are analyzed from the curve variation and amplitude variation of low frequency segments, medium frequency segments and high frequency segments. It's concluded that the curves should not be contrasted vertically, and suggestions for on-site tests are given.