Abstract: Current main reactor thermal-hydraulics system analysis codes are developed based on two-fluid six-equation single pressure model.However,such six-equation model has been proved to be ill posed,which could lead to numerical oscillations.Since two-fluid seven-equation two-pressure model is unconditionally well-posed in all situations,a semi-implicit algorithm based on the finite volume method with staggered grids is developed to solve such well-posed two-pressure model.Then the algorithm is validated on two-phase sedimentation test,water faucet test,and Edwards blowdown experiment.The calculation results show that the proposed numerical algorithm is accurate and robust in solving two-phase flows in the field of nuclear engineering applications.