Abstract: The sealing gasket breakage event of primary coolant loop in China Advanced Research Reactor (CARR) and the event processing by operators of CARR were introduced in this paper. The reason of the event was original installation defects of pipeline flange, and consequences of this event causing loss of coolant were analyzed. It is shown that this event does not affect the radiation safety of workers and cause any impact on reactor and critical system and equipment. All of leakage water is collected in the plant of reactor, and it does not impact on environment. All parameters do not exceed the limits and conditions specified by safety technical specification. The event process data were analyzed and compared with the content of safety analysis report. For the cause of the event and some problems encountered in the process, according to the defense in depth policy, several system improvements and optimizations were elaborated, including gasket replacement, adding flange protective kit, adding leakage signal and strengthening control techniques etc.