The present invention provides a cold-accumulation external cooling system for a direct current converter valve, and an operation method thereof. The system comprises an air cooler (2), a plate-type heat exchanger (3), a water chilling unit, a fixed-frequency cooling pump (6), a buffer tank (7), a thermocline cold-accumulation groove (8), a variable-frequency cooling pump (9), and a cooling water pump (10). The plate-type heat exchanger (3) has a hot side and a cold side, and an improvement thereof is that, the hot side of the plate-type heat exchanger (3) is connected in series with a cooling device (1), the air cooler (2), the plate-type heat exchanger (3), and the cooling water pump (10) to form a circulation loop; and the cold side of the plate-type heat exchanger (3) is connected in series with the thermocline cold-accumulation groove (8) and the variable-frequency cooling pump (9) to form a cold supply loop for a cold-accumulation tank; the water chilling unit, the fixed-frequency cooling pump (6), and the buffer tank (7) are connected in series to form a cold supply loop for the water chilling unit. Compared with the prior art, the cold-accumulation external cooling system for a direct current converter valve and the operation method thereof have a wide range of a cold-accumulation temperature difference; therefore, the system is high-temperature resistant in summer, and is anti-freezing in winter by accumulating heat.