By the cold pressing experiments in a closed container, the stress-strain relationship of the castor seeds was identified and the constructive model was established as well as the model parameters. By comparing the result of experiments with that of the plastic constitutive model based on Doraivelu’s yield criterion, the conclusion was drawn that the equivalent model reflected the hardening law of the castor seeds in practical pressing process well, and with the same axial stress in plastic stage, equivalent strain was maximum 8.3% higher than that of cold pressing while an average height was 5.41%.