Understanding the degradation mechanism of perovskite solar cells (PSCs) is of particular importance to solve their instability issue, which is one of the major hindrances toward commercialization. Here, it is shown that a halide diffusion equilibrium exists at the heterointerface of perovskite devices, which strongly impacts the evolution of device performance. The combined experimental and theoretical studies reveal that halide components diffuse from perovskite to fullerene layers in a p-i-n PSC device and equilibrate with an iodine density of 1018–1019 cm−3 within 80 h under dark aging condition. It is found that there is a strong correction between the device efficiency and halide diffusion equilibrium of PSCs, as the diffused halides can chemically dope the transport layer and result in the nonstoichiometric perovskite surface, leading to both initial enhancement and long-term loss of the photovoltaic efficiency of solar cells. In response to this issue, a predoping strategy is developed to attain the halide diffusion equilibrium once the device is fabricated, thereby avoiding the further halide migration and initial efficiency variations. As a result, the as-prepared PSC achieved an efficiency of 23.13% as well as stable power output under continuous one sun illumination.