Abstract: The inclusion of additives in the biomass pelletizing process is an effective method to improve the pellet quality, which affects the mechanical behavior of the biomass in the densification process significantly. Based on the thick-walled cylinder principle, a mechanical behavior testing system for the biomass densification process was established, and the effects of waste oil additives on the mechanical behavior of the wheat straw and rice straw molding process were studied. The results showed that the peak stress ratios of wheat straw and rice straw ranged from 0.73 to 0.96 and 0.51 to 0.69, respectively. The final stress ratios ranged from 0.46 to 0.68 and 0.38 to 0.55, respectively. The waste oil additive can change the radial stress ratio of materials in the pressing process, but the extent of their influence varies for different materials. With the increase of additive content, the proportion of radial stress increases, but the mechanical properties and its variation trend are anisotropic. There is no definite mapping relationship between stress ratio and product density, and therefore, the product density cannot be determined directly from the mechanical behavior in the molding process.