Aiming at the requirement of thyristor junction temperature prediction in the design stage of ultra high-voltag direct current converter valve, a research method of thyristor temperature distribution suitable for series waterway heat dissipation is proposed. The cooling waterway characteristics of the converter valve module are described briefly, the loss composition of the thyristor level in one cycle is analyzed, and the equivalent loss calculation method is proposed. Combined with the structural characteristics of the valve module, the heat transfer process of the thyristor and the water-cooled radiator is studied, and the thermal resistance model and heat transfer model are obtained. Furthermore, based on the heat transfer model, three kinds of prediction methods for the temperature distribution of thyristor in series with valve modules are proposed. Based on an ultra high-voltag direct current transmission project, the valve module operation test is carried out to monitor the shell temperature of all thyristors and verify the accuracy of the theoretical analysis. The prediction method of thyristor temperature distribution proposed in this paper can provide reference for loss calculation and heat dissipation design of converter valve.