【Objective】To reveal rheological characteristics in the process of castor seed press. 【Method】The stress-time relationship of stress-relaxation process was obtained by stress-relaxation tests with the uniaxial compression equipment. Based on the results and the semi-theoretical and semi-empirical method, a nonlinear rheological model was established using the improved generalized Maxwell model. The parameters of the model were achieved by nonlinear regression method.The calculated values of model were compared with the experimental results. 【Result and conclusion】The results show that the model calculated values matching well with the experimental results, which means the model can simulate nonlinear rheological properties of castor seeds when the strain is over 0.65.