In order to solve the problem of accumulated heat and uneven condition of contact oxygen during straw fermentation process, the study of mixing motion of straw modeled by the discrete element method (DEM) was conducted. The accuracy of DEM models depends on the model parameters. Based on the image analysis, quantitative analysis of experiment and DEM method’s mixing state were conducted. A calibration process was developed to determine the parameter value by experiment of particle’s repose angle and motion. The research results showed that the size of subdomain was related to the particle’s size when using the image analysis of mixing characteristics. The size of subdomain affected the resolution of image analysis and it could be set as six times of particle’s diameter. The order of factors’ influence on the staw’s repose angle was the coefficient of static friction of particle/particle, the particle density, cohesion coefficient of particle/particle and coefficient of rolling friction of particle/particle. The order of factors’ influence on the mixing characteristics of the straw in rotary drum was the cohesion coefficient of particle/wall, the coefficient of rolling friction of particle/wall, coefficient of static friction of particle/wall, coefficient of restitution of particle/wall and coefficient of restitution of particle/particle. The orthogonal test results were compared with the experiment data. The granular material parameters could be used to describe the mixing characteristics of straw.