Abstract: The existed equation of equivalent friction coefficient of V belt has drawback due to the neglect to the deviation of the circumferential friction from the tension of the belt. It’s an effective way to calculate the equivalent friction coefficient by taking the slope angle into account. By adopting the displacement fiber optic sensors in the experiment to measure the radial displacement of V belt, the trajectory of the belt and the value of the slope angle were acquired, and then the equivalent friction coefficient of V belt was obtained. The results showed that from the entry to the exit point when the belt contacted the driving pulley, the slope angle firstly decreased gradually and then increased backward, the absolute value of the slope angle was less than 4.5o when the angular velocity of the pulley was smaller or equal to 1500 r/min; the equivalent coefficient of friction increased from 0.9594 to 1.0596 when the angular velocity of the pulley was 600 r/min, which increased from 0.9529 to 1.0601 when the angular velocity was 1500 r/min. To calculate the equivalent friction coefficient of V belt accurately can make the foundation of evaluating the capacity and service life and reasonable selection of V belt used in the agricultural machinery.