The invention discloses a liquid-surface slag separator. The liquid-surface slag separator comprises an overflow piece and a water pipe. The overflow piece comprises an end socket and a side wall, a cavity with the lower closed side is defined by the end socket and the side wall, and the side wall is provided with overflow weirs. The water pipe is connected to the overflow piece, the water pipe is communicated with the cavity accordingly, and a water inlet of the water pipe is lower than the overflow weirs and higher than the bottom of the cavity. According to the liquid-surface slag separator, based on the nature that impurities float on the water surface, the impurities are filtered two times through the liquid surface tension and automatically enter the water pipe accordingly, and therefore the impurities floating on the liquid surface of a refuelling pool are filtered out. The liquid-surface slag separator is simple in structure and stable in working state, no large fluctuation is generated while the cleanliness of the refuelling pool is effectively guaranteed, and normal operating of a pressurized water reactor is not influenced.