Thyristor is extensively utilized in High Voltage Direct Current(HVDC) transmission and other industrial fields, owing to its large capacity and mature technology. The reliability and safety of the system are directly impacted by the performance of thyristors, which is highly correlated with temperature. The junction temperature(Tj) of the thyristor can be calculated with the on-state voltage Vakon, a temperature-sensitive electrical parameter(TSEP). This paper proposes a technique for online measuring the Vakon, which can rapidly and accurately measure the Vakon without affecting normal operation. Accurately measuring the Vakon can be used to estimate the Tj. The feasibility of the proposed technique is verified by Tj calibration experiment and single-phase half-wave rectifier circuit. The accurate online measurement technology of Vakon has significant importance to ensure the safe and reliable operation of the thyristor and the whole system.