The differential current waveform of the transformer has a large offset due to the influence of non-periodic components and higher harmonics in the first period after the internal fault, which causes the protection to be misjudged. In order to solve this problem, a transformer differential protection simulation model is established in PSCAD. The absolute value of the three-phase differential current is summed as the characteristic current. It is found that there is a significant difference in the waveform discontinuity angle and longitudinal offset of the characteristic current between the magnetizing inrush current and the internal fault current, which can be measured by setting a reasonable threshold above the time axis, and the width of the waveform whose characteristic current waveform is below the threshold is taken as the identification of inrush current and internal fault current. Therefore, a new method based on characteristic current waveform width criterion of Transformer Differential Protection is proposed. The simulation shows that the scheme can effectively improve the speed of protection action and maintain it at around 20 ms, which makes up for the lack of reliability and quickness of differential protection based on the principle of waveform symmetry.