Coupled simulations of system and CFD codes are essential techniques in numerical reactor research. This coupling approach combines the advantages of both codes, enabling high-precision and rapid simulations across multiple scales. It meets the needs of global effect analysis while accurately capturing critical details, thereby providing precise support for reactor design optimization and safety assessments. This study develops a semi-implicit pressure correction iterative coupling method based on domain decomposition, using the commercial computational fluid dynamics software FLUENT and the system analysis code NUSOL-SYS. In this method, a consistent initial pressure value is set at the coupling boundary. The boundary pressure is then adjusted step by step to establish a sensitivity relationship matrix between pressure and mass flow rate. Additionally, the velocity difference across the boundary at each iteration step is used to calculate the coupling pressure correction. Throughout the iterative process, the coupling boundary pressure is continuously adjusted to ensure that the velocities on both sides converge, achieving overall convergence. After verifying the stability of the coupling method through a parallel flow problem, this computational tool is applied to analyze the core inlet flow distribution of the modular small pressurized water reactor ACP100 under a primary coolant pump trip accident. The results show that the system parameters of the coupled code align with the standalone calculations of NUSOL-SYS, while also being capable of simulating local three-dimensional flow phenomena. The proposed method provides valuable reference for multi-scale simulations in the thermal–hydraulic analysis of nuclear power systems.