Abstract: For a gas field project in the South China Sea, simulation and analysis are carried out for the low-pressure charging of the subsea hydraulic system through the comparison of charging time for umbilicals with different sizes, so as to determine the appropriate umbilical size. At the same time, selecting different sizes for the accumulator of the subsea control module (SCM), simulating the opening and closing processes of the 5-1/8 inch gate valve of Christmas tree, we can get the operation duration, the pressure differential of SCM, and the pressure change of gate valve actuator. Comparing the simulation results with the design standard, one can get useful guide for the selection of umbilical size and accumulator capacity, and even helpful information for ensuring the safety and reliability of the subsea hydraulic system in actual operations.