Safety equipment demands that the success criterion of useful equipment, operator-action time window, and the damage state of the reactor core can be defined by thermal-hydraulic (T-H) analysis, which makes a basic critical contribution to probabilistic safety assessment (PSA). PSA has been widely used in the safety evaluation and assessment of nuclear power plants (NPPs). A loss-of-coolant accident (LOCA) cannot be controlled without timely safety intervention. Low-power and shut-down (LPSD) conditions of NPPs can be divided into several plant operating states (POSs) in PSA analysis. After the Fukushima nuclear accident, the topic of station black-out (SBO) has drawn widespread concern. However, some LPSD conditions, which result in severe consequences like SBO, have not drawn widespread attention and are thus analyzed and discussed herein. This paper analyzes a medium-break LOCA (MBLOCA) under a certain LPSD condition for a typical three-loop NPP. A simplified method of simulating and selecting operator-action time of MBLOCA for PSA is developed. The proposed method calculates the time windows for both manually opening the high head safety injection system (HHSI) and secondary depressurizing of the system to keep the core undamaged, which could support building PSA model and human reliability analysis.