To improve the reliability and accuracy of the calculation and analysis for the stress and deformation of the anti-seepage geomembrane at the excavation and filling boundary from the inlet and outlet of a pumped storage power station upper reservoir, the submodel method was adopted to establish a local submodel for the whole model near the boundary of excavation and filling based on the calculation and analysis of the whole finite element model of the reservoir basin backfill. At the same time, this method was used to analyze the effect on reducing the local large strain of geomembrane at the cut fill boundary by the combination of adding additional model area in the overall model and changing the laying modes of geomembrane in the local sub model. The results show that the above measures can reduce local strain so as to meet engineering requirements. Specific laying modes of geomembrane should be analyzed by water head and modulus of the support layer. Keywords: local deformation of geomembrane; submodel method; support layer;pumped storage power station 〖FL