The invention relates to the technical field of containment protection in a nuclear power plant, and discloses a passive water replenishment system for a spent fuel pool in a nuclear power plant. The passive water replenishment system comprises the spent fuel pool and a water replenishment pipeline, wherein the water replenishment pipeline forms a water replenishment joint connected with an external water source; a source taking hole is formed in the sidewall of the spent fuel pool, and is connected with a liquid level monitoring mechanism through an inlet pipeline; the liquid level monitoring mechanism comprises a nuclear grade pressure transmitter for converting the pressure information of cooling water in the spent fuel pool to liquid level information, and the nuclear grade pressure transmitter is connected with an instrument control cabinet for monitoring the liquid level information of cooling water in the spent fuel pool and controlling the water replenishment pipeline to carry out water replenishment. When the spent fuel pool loses forced cooling, the water replenishment pipeline is connected with the external water source, and the cooling water cools a spent fuel, thus avoiding the release of radioactive substances; the pressure information of the cooling water can be converted into the liquid level information by virtue of the nuclear grade pressure transmitter, and is monitored by virtue of the instrument control cabinet connected to the nuclear grade pressure transmitter, and the instrument control cabinet controls the water replenishment pipeline to carry out water replenishment according to the monitored liquid level information of the cooling water.