Abstract: Thermal-hydraulic simulations of pressurized water reactor cores involve solving complex, multi-variable differential equation systems with strong couplings. Owing to the intricate interactions among various physical quantities across different sub-channels, it is challenging to solve the resulting algebraic equation systems after discretization. Traditional iterative techniques frequently encounter convergence difficulties or exhibit numerical instability. To tackle this challenge, this study investigates the block structure of the coefficient matrix and leverages the sparse distribution of different physical quantities within it. By employing a symmetric multiplicative field-split approach, the system is decoupled. The extensive coupled equations are broken down into three subsystems based on physical fields for independent resolution. Additionally, a preconditioning strategy rooted in symmetric multiplicative field-split is developed. Numerical evaluations were performed under four operational scenarios with varying channel scales. In comparison to traditional preconditioning approaches that directly apply Algebraic Multigrid (AMG) or Incomplete LU (ILU) factorization to the entire algebraic system, the proposed method significantly enhances the convergence rate and solution precision of Krylov subspace iterations.