The thermal-hydraulic characteristics of Nuclear Power Plant (NPP) during Steam Generator Tube Rupture (SGTR) accident are of great concern. In this paper, the thermal-hydraulic characteristics of CP300 during SGTR accident with no operator actionsduring the first 30 min are investigated using the best estimate RELAP5/MOD3.4. Modeling and nodalization of CP300 was executed, including the vessel, pumps, pressurizer, steam generations and necessary auxiliary systems. Some main transient parameters were obtained, such as Reactor Coolant Pump (RCP) coolant temperature and pressure, steam generator flow rate and pressure. The calculation results gives the sequences of the NPP during the SGTR as described below. As the tube rupture occurs, the primary pressure drops and secondary pressure increases. When the primary pressure drops down to the set-point of the scram, the control rods drop down and the power of the NPP begins to decrease, causing the primary coolant temperature to decrease. The primary pressure continues to drop. When it drops down to 10.78MPa, the High pressure Safety Injection System is put into operation and the accident is mitigated, even without operator’s actions. In conclusion, the calculated results indicate that the key thermal hydraulic parameters of CP300 during the SGTR accident are in acceptable ranges and the accident is effectively controlled.