Cylinder is used for solid-state fermentation, and the lower crushed straw material layer in the cylinder is lack of oxygen which leads to higher layer temperature, death of yeast and the decrease of fermentation efficiency. Rotating the cylinder will mix the material layers to balance temperature. It needs to forecast the complete mixing time of layers. Firstly, a rotary cylinder was built to analyze the mode of material bed’s rotation, and a model to describe the rotary cycle (T) of material bed was built. As there were too many variables need to be measured, height difference and length of bed’s arc edge which were hard to measure were replaced with easily measured cylinder rotary speed and material volume filling rate based on regression and error analysis. The calculation accuracy of the simplified formula was higher than 95%. Secondly, based on image analysis method and VB programming, complete tests were done to get the complete mixing time (τG) of upper and lower layers at different filling rates (0.2, 0.4, 0.6) and rotary speeds of cylinder (0.9, 4.5, 8.0 r/min). τG was divided by T to get rotation number of material bed (NR) when two layers were completely mixed. The results showed that NR had little relationship with rotary speed of cylinder and had linear relationship with filling rate. Based on regression and error analysis, a formula was built to calculate NR at different filling rates. Multiplying T by NR can derive a formula which was used to forecast τG of upper and lower layers. The calculation accuracy of the formula was higher than 90%. The way to get such a formula on other cylinders was also introduced. The result is helpful on designing and operating of cylinder in order to improve fermentation efficiency.