Transition metal selenide is a typical pseudocapacitive material used in supercapacitor electrodes, but its practical application is restricted due to its insufficient utilization rate of active sites and slow kinetics of redox reaction. Defect engineering can effectively adjust the electronic structure, provide additional intercalation sites, and realize the regulation of energy storage mechanism within the lattice. At present, heteroatom doping is an effective strategy that can provide abundant vacancy defects by introducing heterogeneous atoms, thereby achieving significantly improved electrochemical performance. Based on this, NiCo₂Se₄ electrode materials with Se vacancies were prepared by P doping for supercapacitors. The double-effect synergy of P doping and vacancy engineering can effectively change the electronic structure and improve the adsorption capacity of ions, which are more prone to redox reactions. The assembled hybrid supercapacitor (NiCo₂Se_x-P₁//AC) has 90.2% capacity retention and 100% coulombic efficiency after 10 000 consecutive charge–discharge cycles, and it exhibits a high energy density of 94.6 W h kg⁻¹ at a power density of 799.9 W kg⁻¹. This provides an effective strategy for constructing high-performance supercapacitors.