This paper presents a new on-site testing method for IGBT power loss in order to overcome the shortcomings of traditional method with large error or difficulty in engineering implementation. The method can be applied to verify the performances of converter or diagnose the early fault of power modules by measuring the losses characteristic of IGBT module on-site by discharging DC capacitor. During the test, a DC capacitor discharging circuit is constructed by proper control of IGBTs inside the converter. Experimental study is carried out on a 380 V/10 kW variable frequency drive (VFD) with Infineon IGBT module FFS0R12RT4 in this paper and the results prove the feasibility and effectiveness of the method. The proposed method takes use of the existing DC capacitor and load inductance to perform the on-site losses testing when system is shutdown, so that it has advantages of no hardware modification, safe, simple and cost-effective.