Abstract: The environmental qualification test facility of Nuclear Power Institute of China, cannot meet the requirement of the environmental test of loss of coolant accident (LOCA) in the third generation NPPs, for the issues such as the inadequate capability of temperature rising and the uneven temperature distribution in the test chamber during the thermal shock. To solve these issues, the RELAP5 code was used for the numerical analysis for the design improvement of the test facility. Based on results of the numerical analysis, the facility was improved. The test results from the improved facility demonstrated that the capability of temperature rising in the test chamber during the thermal shock and the uniformity of the temperature distribution was largely improved. The new facility meets the requirements of design base accident (DBA) environmental test for the third generation NPPs.