Abstract: According to the improvement proposal for the residual air criteria of the primary loop dynamic venting, the pressure prediction method for the main primary nodes and the judgment method for the two-phase coolant flow are investigated. The thermo-hydraulics model for the primary loop and its auxiliary systems are proposed. When the residual air criteria is raised to 24 normal cubic meters, the startup transient process of reactor coolant pumps are simulated with the models, to obtain the pressure of the main nodes in the primary loop. Based on the gas resolution and released model, the change trend of nitrogen saturation solubility and oxygen saturation solubility are obtained. The results indicate that the pressures of the main nodes are within the normal operation limit; the nitrogen and oxygen cannot release from the coolant and cannot form a two-phase flow. Therefore, in terms of the coolant flow characteristics, the improvement for the residual air criteria to 24 normal cubic meters is feasible.