In this paper, a fiber sensing system combined with fiber loop ring down (FLRD) technique to detect aflatoxin B1 (AFB1) in food is proposed. The single-mode fiber (SMF) taper was set as the sensor head, and the waist diameter of it was determined to be $17.1~\mu \text{m}$ by the refractive index (RI) measurement experiments. When a light pulse enters and circulates in the fiber loop, it experiences a little loss each time as it passes through the sensor head. The ring-down time of the system would be altered by intensity loss near the sensor head, which is related to the concentration of the AFB1 solution. The experimental results showed that the sensitivity of AFB1 was - $0.755~\mu $ s/( $\mu \text{g}$ /ml) in the concentration range of 0- $5~\mu $ g/ml with a precision of $0.016~\mu \text{g}$ /ml. Compared with other AFB1 sensors, the system has the advantages of simple structure, low cost, fast response and insensitive to light source fluctuation.