The invention provides a chemical dosing and liquid replenishment device and method for a nuclear power plant containment filtration and discharge system. The chemical dosing and liquid replenishmentdevice can be movably connected with the containment filtration and discharge system, and includes a liquid replenishment tank for storing a mixed chemical solution and a stored qualified medicine solution returned from a washing unit and a pipeline assembly for mobile liquid replenishment for the washing unit when a liquid level or chemical concentration signal received in the washing unit in thecontainment filtration and discharge system is lower than a preset threshold value. According to the chemical dosing and liquid replenishment device and method for the nuclear power plant containmentfiltration and discharge system, the problems that in the prior art, the anti-seismic effect is poor due to the integrated arrangement of the chemical dosing and liquid replenishment device and the nuclear power plant containment filtration and discharge system, and the process of liquid distribution and liquid replenishment is complex are solved; and by adopting the chemical dosing and liquid replenishment device, the site accessibility is good, the optimal and most convenient liquid replenishment location can be selected in a plant space, the long-term liquid replenishment operation of thecontainment filtration and discharge system after a serious accident is facilitated, and the long-term cooperation with emergency measures inside and outside the plant and water replenishment water sources is facilitated.