Abstract: The pressurized thermal shock (PTS) analysis of a reactor pressure vessel (RPV) with a semi-elliptical surface crack was performed under loss of coolant accident (LOCA) transient. The finite element method (FEM) was adopted to calculate the thermal-stress responses. Stress-intensity factor (SIF) was calculated by influence function method. Stress distributions in the base material and bead welding layer were decomposed, so that the difficulty of stress fitting caused by existence of bead welding layer was conquered. A fracture analysis program was developed in present research to analyze the RPV integrity under LOCA. The results show that during the whole LOCA transient, SIF of RPV crack does not exceed fracture toughness, which means RPV structure is safe. The present research will provide a theoretical guidance for RPV integrity assessment under PTS.