The invention discloses a detection system for optic-fibre leakage at the bottom of a normal atmosphere storage tank, which comprises a plurality of sensing systems. lasers are adopted for the plurality of sensing systems as incoming ray sources which are connected to optical intensity style optical fibre sensing probes via optical fibres and are divided into a plurality of routes via optical-fibre couplers and reach a plurality of sensing probes, reflex detected by the sensing probes enters PIN tubes and are connected with a data collection card via a photoelectric conversion circuit and an amplifying circuit, the data collection card is connected with an industrial computer for data analysis, processing, displaying and recording. The detecting system alarms by software automatically. The system has high precision detection, rapid speed, timely alarm, accurate location, strong anti-interference capability, high safety and convenient system operation, can display where the storage tank leaks and how long the storage tank leaks in real-time by curve mode and digital mode and has aural and visual alarm. Due to the flexible system communication mode, the invention can realize off-site search and monitoring, thereby accelerating processing speed, decreasing economic loss and reducing environment pollutions.