Abstract: Abstract: Biomass briquetting is one of the key technologies to solve the problem of its collection, transportation and storage. Meanwhile biomass pellet, as a renewable energy, can be used instead of coal for heating, electricity, etc. Study on the mechanism of biomass briquetting is the basic way to develop new forming methods and equipments, and to improve productivity greatly. The research in this paper is about a thermo viscoplastic constitutive model for the biomass briquetting mechanism based on endochronic theory. Straw mainly consisting of cellulose, hemicellulose and lignin, is a typical example of dissipative material. Biomass briquetting process is a mixed process of extrusion flow and interstitial flow. During the process, a large quantity of heat is generated by internal friction. The increase of temperature causes the lignin to soften which provides cohesive force for fiber particles briquetting. To verify the effect of lignin and temperature on the straw plastic deformation, stress-strain test and briquetting temperature test were run on the straw with different lignin mass fractions. Three kinds of biomass raw materials including corn straw, wheat straw and rice straw were the study objects in this paper. Then, these raw materials were added with additional lignin respectively in accordance with the following mass fractions 10%, 20% and 30%. The results show that the increase of lignin mass fraction can reduce stress and temperature obviously. The maximum stress of straw with additional mass fraction of 20% is reduced by about 10 MPa compared with raw materials′，and the maximum temperature is reduced by about 10%. Consequently, lignin and temperature have a crucial effect on straw biomass briquetting process which is thermo viscoplasticity process. The constitutive model should reflect the internal characteristics of materials, especially the characteristics of lignin. In order to consider the internal variables of straw, endochronic theory applicable to the dissipative materials was adopted to build the thermo viscoplastic constitutive model. First, endochronic time which replaced Newton time was defined to describe thermo viscoplasticity deformation. Strain, strain rate, temperature and lignin mass fraction were the main internal factors to formulate the briquetting mechanism. Second, the endochronic constitutive equation of straw briquetting was derived according to thermodynamic law and clay rheology. Third, tests were planned to analyze the influence of lignin mass fraction and temperature on straw forming process. The optimal lignin mass fractions of corn straw, wheat straw and rice straw are respectively 29%, 33.5% and 34.3%. The optimal temperature is between 100 and 115℃. Strengthening function was deducted based on the test data and Johnson-Cook model. Finally, verification test was designed upon GDS consolidation test system. Coefficients of constitutive equation were got by numerical analysis. Compared with test data, the thermo viscoelasticity constitutive model based on endochronic theory has its extensive applicability. When the temperature is between 100 and 115℃ and the strain rate between 1×102 and 1×103 s-1, corn straw with lignin mass fraction of 29%, wheat straw with lignin mass fraction of 33.5%, and rice straw with lignin mass fraction of 34.3% have the best plastic briquetting performance. This study results provide an operable material model for the finite analysis and an important theoretical basis for equipment structure design.