The utility model discloses an on -line monitoring device based on ultrasonic wave quantitative determination SF6 gas leakage, including CPLD signal processing module, CPLD signal processing module connects ultrasonic transducer module, comparator module and ARM module respectively, the signal is wherein gone the same way through ultrasonic emission sensor a, background gas cavity, ultrasonic wavereceiving sensor a, filtering amplification module an and the connection of comparator module to ultrasonic drive transducer module, another way signal of ultrasonic drive transducer module is surveyed gas chamber, ultrasonic wave receiving sensor b, filtering amplification module b and comparator module through ultrasonic emission sensor b, quilt and is connected, the problem of the signal attenuation s of data in the transmission course etc. Lead to last measured data precision of gathering not high is solved.