The invention provides a gaseous tritiated water online detection system based on a plastic scintillator. By using the system, the activity of gaseous tritiated water can be measured in real time. According to the system, a pump is used for extracting a gaseous tritiated water sample from a sampling area and sending the gaseous tritiated water sample into a purification separator, the gas subjected to purification and separation is sent into a detection region, and the activity of tritiated water can be obtained in real time. The system comprises a sampling unit, a purification and separationunit, a measurement unit and a control unit. The gaseous tritiated water online detection system can measure gaseous tritiated water samples in various activities in real time, and has the advantagesof being short in measurement time and rapid in reaction. The problem about measuring tritium in real time is solved. The system further has the advantages of being easy to operate, capable of achieving software control and the like.