Relaxation behavior of natural fiber reinforced polymer composites exists in both solid and molten state and profoundly affects the control and optimization of processing parameters and dimensional accuracy of final products. The aim of this study was to establish an experimental estimating method to characterize relaxation behavior of polymer composite with traditional torque rheometer and investigate the effects of fiber concentration, dimension, and processing parameters on the relaxation behavior. Relaxation time of hemp fiber reinforced polypropylene composite were obtained by fitting the experimental torque data and corelated to the buckling deformation of packaging boxes manufactured by injection with the same injection parameters. The results show that all the increasing concentration of hemp fiber, higher rotor speed, and higher temperature can significantly decrease the relaxation time and result in a stable and better dimensional accuracy of injection packaging boxes. The finer fibers with the same weight concertation can bring an obvious reduction in relaxation time and buckling deformation. The proposed method for estimating relaxation behavior of polymer composite is a feasible and cost-effective way to investigate the viscoelastic properties of polymer composites close to the real processing conditions. And it can be widely used in optimization of processing parameters and performance of polymer composite products.