Abstract: Abstract: Grass used as a high-quality animal feed has been grown in large scale in China. It is very important to study its stress relaxation characteristics to reduce the power consumption of round bales and improve the quality of bales. In order to understand the stress relaxation characteristics of alfalfa in course of rotary compression, a rotary compression test platform was designed using a steel-roll round baler. The radial forces of steel rolls to the bale during rotary compression were analyzed. Because of the different location of the steel roller of the baling chamber, the radial force of the steel roller to the bale is also different. The radial stress of the steel roll of the upper part of the baling chamber to the bale during rotary compression was obtained through the combination of the wireless resistance strain gauge and the ANSYS simulation. On the basis of the analysis of the stress relaxation test curve and the model principle, it is determined that the stress relaxation behavior of herbage can be simulated by a generalized Maxwell model composed of 2 Maxwell elements and 1 spring element in parallel. Residual stress method is used to get stress relaxation time and equilibrium stress rheological parameters in the model, which were 76.92 s, 23.25 s and 3.65 kPa respectively. The coefficient of determination (R2) was used to evaluate the model fit to the experimental data. The parameters of stress relaxation were analyzed. The moduli of the 2 Maxwell elements were E1 = 1.88/ε1 and E2 = 6.1/ε2. The modulus of the material was not changed during the relaxation process. The stress relaxation times were T1 = η1/E1= 76.92 s and T2= η2/E2 = 23.25 s. It reflected the relaxation speed of the material and the stress relaxation time of each Maxwell element is independent of each other. The initial stress for stress relaxation was 11.63 kPa (σ = σ1 + σ2 + 3.65), and the equilibrium stress was 3.65 kPa. This study provides a theoretical basis and technical support for the further exploration of the relationship between feed volume, compression frequency, water content and rheological properties of materials and for the design of energy saving baler.