As the most severe assessment method in transformer commissioning testing, partial discharge testing can effectively detect insulation faults inside transformer. In this paper, the adjustable reactor has been firstly proposed and used in the partial discharge commissioning testing of the converter transformers in Jinping-Sunan ±800 kV DC transmission project. The capacitive reactive power of the converter transformer under testing has been analyzed and the inductive compensation has been calculated. The reluctance of the adjustable reactor can be adjusted smoothly, therefore the inductive compensation can be changed continuously in real time and the under-compensation or over-compensation problems of fixed reactor can be avoided. The testing method is validated in the field commissioning testing in Jin-Su UHVDC project and the filed partial discharge testing is completely successful.