Abstract: Based on the four-element model,the equation of the stress relaxation process is developed using the principle of Boltgation superposition in this paper.Stress relaxation experiments of air jet vortex spun and ring spun yarns made of naturally colored cotton are measured.The regressive equation of the stress relaxation of yarns is calculated using Origin 7.5 based on the model of the stress relaxation,and the stress relaxation mechanism of air jet vortex spun yarn is explained.The results show that the theoretical value is conformed with the experimental results very well,and this model can forecast the relaxation properties of yarns.The relaxation time of air jet vortex spun yarns is relatively short,so the elasticity is smaller.Through predicting the relaxation property of the yarn spun with naturally colored cotton,the hand feeling of its product can be improved.