It is of great importance for the thyristor valve in D. C. high-voltage transmission to be chilled reliably and safely. Considering that the water cooling technology is usually used in practice, this paper discusses the whole process about water cooling in D. C. high-voltage transmission based on some real company's application background. By using the Fluent software, the thyristor's water cooling model of the internal cooling system is simulated through a simplified model, Based on the simulation results, the influence of some relative parameters to the water cooling effect is analyzed, which can provide the reference date for applying the water cooling technique in the application.