An accurate and reliable control method of irregularity in yarn fineness is presented. It's based on the theory of the influence of cotton fiber length distribution on the evenness of yarn, and it improves the predictive model of irregularity in yarn fineness. Under the producing condition of controlling, we set a value or value range of predetermined term of or/and predictive model. And according to the given material and yarn parameter and predicted yarn fineness, it can be calculated that the parameter range of material fiber which is to be controlled. Finally, the yarn is span according to the practical production requirement, come up to the desired yarn irregularity, and the control of irregularity in yarn fineness is completed. The built predictive model is consistent with practical production. The control method on the basis of the model is precise and convenient. It's also avoiding such problems as either works and material wasting or poor quality of yarn which happened in controlling the irregularity of yarns by experience. The control of irregularity in yarn fineness becomes convenience and practical by using this method which is with much higher manipulability.