Neutral Beam Injection (NBI) is a key plasma heating method in magnetically confined fusion devices, the core performance of which lies in the ability of the ion source to extract intense ion beams. To improve the performance of the ion source, the plasma ion density near the extraction region of its discharge chamber was enhanced. In this study, an innovative scheme was proposed for a hot-cathode ion source, in which a biased electrode was installed above the extraction region to optimize the plasma distribution through an electrostatic confinement effect. A theoretical model based on global balance was first established, revealing that a positively biased electrode formed a potential barrier that confined low- and medium-energy electrons and prolonged their path length, thereby increasing the ionization efficiency. The ion density was theoretically predicted to increase proportionally. To verify this method, a plasma fluid model was constructed, and the plasma transport equation and Poisson’s equation were solved self-consistently using plasma simulation. The simulation results indicated that after applying a bias voltage of +30 V, the plasma potential distribution near the extraction region was effectively elevated, which was consistent with the theoretical predictions. This study confirms, at both theoretical and simulation levels, that the electrostatic field generated by a biased electrode is a feasible technical approach for enhancing the plasma density in hot-cathode ion sources, providing important theoretical basis and design guidance for the optimal design of future high-performance neutral beam injection systems.