Abstract: Under the loss of coolant accident in nuclear power plants, the blockage of sump strainers may be resulted from chemical effects, which will affect the recirculation function of the emergency core cooling system or the containment spray system, disabling the cooling of the core and containment, and threatening the safety of nuclear power plants. Study on the effects of potential chemical products on sump strainer head loss after the loss of coolant accident in Qinshan PhaseⅡExtension Project was conducted. The results showed that the alumina-bearing material and the insulation material released aluminum and silicon in the sump environment, and that aluminum and silicon formed chemical precipitation and deposited on the debris bed. This can result in the blockage of the debris bed and the reduction of the porosity, and increase the head loss of the sump strainer. There are chemical effects after loss of coolant accident in Qinshan Phase Ⅱ Extension Project, and it should be considered in the sump strainer performance evaluation and downstream effect analysis.