The main steam system is an important system connecting the primary and secondary loop of the nuclear power plant. It transports the steam generated by the steam generators to the steam turbine to drive the turbine to turn and do work. The main steam isolation valve (MSIV) is important safety devices in nuclear power plant. Its safety function is to automatically shut down under the accident condition of large break in the main steam pipe or main water supply pipe in order to limit the steam leakage and mitigate the consequences of the accident. The main steam partial shutdown test is a test to periodically verify the operability of the MSIV during the power operation of the nuclear power plant. The test period is usually once a month. However, during the test, there may be shutdown events caused by the mistakenly close of the MSIV, so the test is a high-risk periodic test. At the same time, due to the limitations of field condition and space, there are so many industrial risks and the risk of accidental collision leading to shutdown during the test. At present, the probabilistic safety analysis (PSA) is widely used in operation guidance, maintenance rules and many other fields of nuclear power plants. PSA has gradually developed into an important tool for safety evaluation and decision making. With the development and improvement of PSA technology, risk-informed safety management has been widely used. The this paper, the use of risk-informed PSA method to demonstrate the extension of periodic test period will be introduced. And taking FJS Nuclear Power Plant as an example, the extension of the periodic test period of the partial closing test of the MSIV is demonstrated.