Palladium diselenide (PdSe₂) is acknowledged as a potential electrocatalyst for hydrogen evolution reaction (HER), but its large-scale basal plane is catalytically inert, which enormously limits its application in electrochemical efficiency. Here we employ the phase interface in PdSe₂ to activate sites on the basal plane and improve the HER activity. Five P/T interface structures in PdSe₂ are constructed and optimized by density functional theory (DFT) calculations. The interface boundaries consist of triangle, pentagonal and hexagonal rings with the formation of more Se-Se bonds. The structural stability and HER activity for these interface structures are evaluated, and the optimal sites for HER are confirmed, which are mostly located on Se atoms. Our proposed strategy proves the formation of interface in PdSe₂ could successfully enhance its HER performance and could be also applicable to the other two-dimensional materials.