Abstract: A new approach for online leakage monitoring of double-bottom plate structure of vertical tank is presented in the light of the deficiencies in common methods, and a testing system based on PLC and corresponding sensors is set up. This system is proved to be reliable and sensitive through leakage tests with 93^# gasoline, diesel oil or water as a medium. Research on the impacts of leakage position and leakage rate on the leakage diffusion with 93^# gasoline as a medium shows that for a constant leakage rate, leakage diffusion occurs most quickly from tank bottom center with a minimum time to be detected, and it would take more time to detect the leakage diffusion from tank edges. For leakage from a same source, leakage volume fraction in a pipeline would increase with the leakage rate. This system is proved to be reliable and sensitive, and could be directly applied to the in-situ monitoring of vertical tank with double-bottom plate structure.