Abstract: Timely detection and accurate positioning of the partial discharge(PD) source inside the transformer can help detect internal insulation defects as early as possible, reduce the risk of transformer explosion, and thus improve its operational safety and reliability. However, due to the large volume and complex structure of transformers, it is extremely difficult to accurately locate the PD source. A localization method of PD source that considers non-line-of-sight(NLOS) propagation of electromagnetic wave is proposed, based on the traditional time difference of arrival(TDOA) localization method. This method parameterizes the positioning error caused by electromagnetic wave NLOS propagation. Moreover, the strategy of utilizing more than four sensors is adopted to reduce the possible effect of overcorrection on NLOS propagation. Firstly, the derivation and implementation process of the proposed method is introduced from the perspectives of mathematical model and geometrical model. Subsequently, the experimental verification is conducted on a 35 kV transformer with six ultra-high frequency(UHF) sensors. The experiments found that the proposed localization method had a better calculated accuracy and stability, the obtained minimum calculated error is 10.88 cm, the calculated accuracy can be improved by 82.04% and 78.94% respectively with six sensors than four and five sensors arrangement.