Abstract: This study addresses the challenge in the JJG 620 Critical Flow Venturi Nozzle verification regulation for large-throat Venturi nozzles (throat diameter ≥ 50 mm), where, after passing the geometric inspection, the discharge coefficient cannot be calculated due to unclear calculation paths for key parameters, such as the Reynolds number, in the empirical formulas. To resolve this, two methods for calculating the discharge coefficient based on geometric dimensions are proposed: the ideal mass flow method and the mass flow iterative method. By analyzing these two methods, a rigorous and operable computational chain is established to directly determine the discharge coefficient from the nozzle’s geometric parameters, and the determination methods for key parameters not addressed in the regulation are clarified. Using two nozzles with nominal throat diameters of 51.960 mm and 73.460 mm as case studies, the results show that the differences between the ideal mass flow method and the mass flow iterative method are minimal when calculating the discharge coefficient for these two nozzles, and both methods can calculate the discharge coefficient after geometric inspection. The example calculations and verifications using both methods show highly consistent results, providing reliable technical support for the practical application of the regulation.