In order to study the method to determine the blended ratio of the soybean protein fibers, the calculation of the mean and standard deviation of the blended ratio of soybean protein fiber/polyester blended yarn by the statistical theory and density gradient method is discussed in this paper. The method can derive not only the mean but also the standard deviation of the blended ratio of the soybean protein fiber/polyester blended yarn. The control range of blended ratio of roving and ring yarn is (63.63%, 65.32%) and (64.16%,66.17%).The control range of the standard deviation of roving and ring yarn is 2.89% and 3.56%. The blended ratio of the ring yarn by the chemical dissolution is 64.5% at the same time. The error of the two methods is only 1.1%.