The junction temperature (Tj) of the thyristor is closely related to its operating conditions, and Tj significantly affects the performance and reliability of the thyristor. The Tj is essential information for health assessment. This paper proposes an online junction temperature measurement method for high-voltage and large-power thyristors with the press-pack structure. Theoretical analysis shows that the gate voltage VGK of thyristor during the trigger state can be utilized as a temperature-sensitive electrical parameter (TSEP) for online Tj monitoring, and the influence of pressure variation on Tj extraction is analyzed. Additionally, the proposed circuit can drive the thyristor and measure the VGK. VGK can be used for online monitoring Tj fluctuations in the thyristor based on the long-term load test.