The invention discloses a passive nuclear power station LOCA (Loss of Coolant Accident) accident mitigating system belonging to the technical field of nuclear power station safety. Memory alloy is connected with a four-bar mechanism and a valve in series to form a corresponding memory alloy intelligent switching device by utilizing the characteristic that the memory alloy generates contraction deformation at certain temperature, and the memory alloy intelligent switching device is installed at a proper position in a containment and executes a corresponding brake action under the condition that high-temperature and high-pressure steam is filled in the LOCA accident containment. The strong moisture absorption characteristic of lithium bromide solution is utilized to absorb the high-temperature and high-pressure steam in the containment, thereby achieving the goal of mitigating the accident. Lots of passive modes of natural circulation, four-bar mechanism power transmission, pressure action and the like exist in the system. The system mainly adopts the passive automatic operation and has the advantages of convenient implementation, simple control and high reliability, safety and stability.