According to severe false alarm problem of the phase sensitive optical time domain reflectometer (Φ-OTDR) optical fiber distributed disturbance sensor system, based on multi-channel monitoring system with Φ-OTDR scheme, the improved D-S evidence theory was applied to the Φ-OTDR three-channel monitoring system to reduce false alarm rate. Each output data was fused using improved D-S evidence theory information fusion algorithm. Then the fusion result was compared with the alarm threshold to determine whether the alarm occurs. The experimental results show that Φ-OTDR three-channel monitoring system based on the improved D-S evidence theory can not only correctly judge the disturbance signal in the three-channel monitoring scheme, but also decrease the leak alarm rate from 3% to 1%, and the false alarm dropped from 3.3% to 0% within 72 hours, improving the system alarm performance.