The invention relates to a power plant seawater once-through cooling system. The power plant seawater once-through cooling system comprises an intake canal with a water inlet, a drainage canal with a water outlet, a circulating water pump, a mixing water pump, a heat exchange device and pipelines; the intake canal, the circulating water pump, the heat exchange device and the drainage canal are sequentially connected through the pipelines to form a main cooling system body, the intake canal, the mixing water pump and the drainage canal are sequentially connected through the pipelines to form a mixing cooling system body, and a first valve is connected between the mixing water pump and the drainage canal. According to the power plant seawater once-through cooling system, the main cooling system body and the mixing cooling system body are arranged, original seawater directly transported by the mixing cooling system body and thermal discharge water output by the main cooling system body are mixed in the drainage canal, and therefore the temperature, discharged to a sea, of the thermal discharge water is reduced; the temperature, entering the sea, of the thermal discharge water can be reduced without needing to arrange a cooling tower, the thermal pollution of the thermal discharge water of a power plant is reduced, the plant area is greatly decreased, the initial investment is reduced, the energy consumption is reduced, and the economic benefit is improved.