The insulated-gate bipolar transistor (IGBT) in a modular multilevel converter (MMC) mostly operates under high voltage conditions, and a large-range sensor is required to measure the collector-emitter voltage. However, a large measurement error will occur and this restricts the accurate calculation of the on-state loss. Therefore, an online calculation method of on-state loss based on the voltage-current characteristic curve for the IGBT in the MMC sub-module is proposed. First, two-dimensional and three-dimensional fitting of the relationship model between on-state voltage drop, collector current and junction temperature is achieved based on the characteristic curve parameters of the IGBT and diode. Second, the switching mode of the device in the unit current cycle is analyzed to realize the expression of the on-state loss. In addition, based on the related theory of electric-thermal comparison, an equivalent thermal network model of the IGBT is constructed. Then, comprehensively considering the device current, turn-on signal and case temperature, etc., the junction temperature is corrected by feedback, and the online calculation method for the IGBT on-state loss is further formed. Finally, the effectiveness of the proposed method is verified by experiment. This work is supported by the Key Project of National Natural Science Foundation of China (No. 51637007).