Abstract: 【Objective】 To construct cotton straw degrading strains and optimize their solid fermentation conditions. 【Method】 The cellulose degrading strains were screened based on carboxymethyl cellulase (CMC) and filter paper activity (FPA), and mixed with bacteria strain to adjust the fermentation quality and identify the symbiotic strains mixing ratio, then, the effect of the initial moisture content, inoculation amount, fermentation time and their interactions on cotton stalk cellulose degradation rate were evaluated by using a response surface method. 【Result】 24 strains of cellulolytic strain bacteria were obtained, among which, the maximum ratio of the diameter of one transparent circle to the diameter of colony was 6.20, the enzyme activity of FPA was 9.699 U/h, and the enzyme activity of CMC was 10.435 U/h. The strain was identified asbacillus subtilis, which was mixed with J1 (Candida utilis), J2 (Lactobacillus plantarum), J3 (Bacillus licheniformis) to construct composite strains at the ratio of 1∶1∶2∶1 for fermentation. The results of variance analysis showed that the regression equation was significant (P < 0.0001), inoculation amount was 15.0%, fermentation time was 35 days, water content was 80.0%, cellulose degradation rate was 35.91%, the effect of inoculation amount on cellulose degradation was the most significant. 【Conclusion】 In this experiment, the cellulose-degrading strain of cotton straw was constructed, and the optimum conditions of solid fermentation were determined, which provided the experimental and theoretical basis for the feed conversion of cotton straw.