Abstract: Based on the one-dimensional steady-state separated flow model and low-velocity subcooling model of net void generation（NVG） point, a method to study the characteristics of the natural circulation flow in the annular channel under condition of in-vessel retention has been established. The results are compared with the ULPU-V experiments which measured the natural circulation flow rates to verify the reliability and correctness of the numerical calculation model established in this work. The effects of configuration parameters and thermal-hydraulic parameters on the natural circulation for in-vessel retention of AP1000 are analyzed. The results show that the water subcooling, flow channel size, flooding levels, inlet area and loss coefficients of riser outlet have a dominant effect on the natural circulation flow rate. And the natural circulation flow rate shows various variation tendencies along with those parameters due to the driven forces and flow resistance.