In order to study the compression stress relaxation characteristics of cotton stalk particles, 22 different varieties of cotton stalk particles have been collected from different growing areas and environments and crushed by 40 mesh sieve, and then stress relaxation test have been implemented on them by RGM-6010 universal material testing machine. The stress relaxation curve of cotton stalk particles is fitted to the three-element generalized Maxwell model, the five-element generalized Maxwell model and the Peleg model. This article uses the obtained information and the fitting coefficient of determination as the evaluation indexes to establish the powder cotton straw stress relaxation constitutive equation. Results show that the stress relaxation curve showed an exponential, and had equilibrium stress. The five-element generalized Maxwell model is best to depict stress relaxation characteristics of cotton stalk particles and the fitting coefficient of determination, balance elastic modulus, the first stress time, and the second stress time are 0.999, 4.477 MPa, 4.39 s and 48.65 s respectively. Therefore, stress relaxation characteristics of cotton stalk particles were depicted best by the five-element generalized Maxwell model.