One of the most important issues for the assessment of life time extension of a nuclear power plant is whether the Reactor Pressure Vessel (RPV) can maintain structural integrity under Pressurized Thermal Shock (PTS) phenomenon. The most critical scenario takes places during cold water injection in the cold legs after a postulated Loss of Coolant Accident (LOCA). In the present study, a deterministic method has been conducted. The thermal-hydraulic system analysis program CATHARE, the Computational Fluid Dynamics (CFD) software STAR-CCM+ and the mechanical structure analysis software ABAQUS are used to simulate the Small Break and Intermediate Break Loss of Coolant Accident (SB-LOCA and IB-LOCA). As a result, the SB-LOCA and IB-LOCA accident transients are both simulated and the thermal stratification during the injection is demonstrated. The analysis results give the lowest temperature of SB-LOCA under vertical injection. Because of the bigger injection flow rate, the thermal shock is severer in IB-LOCA than SB-LOCA. According to the mechanical structural analysis results from ABAQUS, the thermal stresses are mainly concentrated on the both sides of the bottom of the inlet nozzle and the beginning of the downcomer of Reactor Pressure Vessel (RPV).