Abstract: Accurate characterization of stress relaxation in loose materials can greatly contribute to reducing energy consumption of briquetting, while improving the quality of briquettes in modern agriculture. Thus, rheological tests and data fitting were often utilized to optimize the parameters in different models. However, a relatively large number of parameters are contained in the current stress relaxation models, failing to fully describe the permanent deformation. This study aims to improve the fitting accuracy of the stress relaxation model, while reducing the number of model parameters, particularly for better representing the parameters related to the permanent deformation of loose agricultural materials. A simple stress relaxation model was also proposed using fractional calculus to describe the stress relaxation behavior of cornstalk mixing with potato residues. Cornstalk and potato residues were mixed in a weight ratio of 1:3. The cornstalk was cut into pieces < 3mm and dried naturally to a moisture content of (4±0.1)%, whereas, the potato residue was dehydrated to a moisture content of (65±1)%, and then collected from a starch processing factory. A stress relaxation test was conducted to acquire stress-time curves under five compression densities (700-1 100 kg/mm3) using a self-developed adjustable compression device mounted on a universal testing machine. The stress-time curves illustrated that there was obvious stress relaxation behavior in the mixture of cornstalk and potato residues when compressed. A Generalized Maxwell model and a fractional model were proposed to evaluate the parameters using curve fitting and regression analysis. The results indicated that the fractional model presented a better fitting accuracy than the Generalized Maxwell model, where the coefficients of determination were between 0.996 4-0.999 5. Moreover, there were only two undetermined parameters (K and β) in the fractional model, less than the parameters (elastic modulus E1, E2, Ee and viscosity coefficients η1, η2) in the Generalized Maxwell model. Thus, the fractional model was expected to more accurately and briefly represent the stress relaxation behavior of the mixture, compared with the Generalized Maxwell model. Additionally, correlation analysis showed that K was significantly correlated with E1, E2, Ee, η1, η2, and thus K was expected to serve as a key coefficient for the viscoelastic characteristic of mixed materials. In the fractional order, β was significantly correlated with the ratio of equilibrium modulus Ee to total modulus E0, and thus β was used to describe the proportion of residual elasticity with respect to total elasticity, representing the permanent deformation of mixed materials. The relaxation ratio Sr and relaxation rate St were also calculated to verify the physical meanings of K and β, together with the connection with K and β. It was found that β was significantly negatively correlated with , whereas, K was significantly positively correlated with St. It further proved that the fractional model can be used to describe the permanent deformation degree of mixed material after stress relaxation. Consequently, the fractional model successfully represented the stress relaxation behavior of cornstalk mixing with potato residues, where the obtained parameters fully represent the degree of permanent deformation. The finding can provide both potential theoretical and practical significance to analyze the briquetting mechanism, and improve the briquetting process of loose agricultural materials.