As the capillary rheometer is not appropriate for measuring the rheological properties of the composites of plant fibers and polymers,a new measurement method using the torque rheometer was proposed and was then used to measure the rheological properties of polybuthylene succinate （ PBS） composites reinforced with sisal fibers （ SF） in different mass fractions. Then,the length-to-diameter ratio of sisal fibers in the composites was measured,and the effects of SF content and rotor speed on the length-to-diameter ratio of SF and the flexural strength of the composites were analyzed. The results show that the shear-thinning phenomenon of the composites becomes more significant with the increase in SF content due to the disentangling,breaking and orientation of SF.