The experiments for stress-strain and lateral pressure of sunflower seeds by uniaxial pressing were carried out. The results show that sunflower seeds conform to the model of three power curve. By using of Kuhn's yield criterion, plasticity constitutive equations were developed. Sunflower seed strain of theoretical and experimental value of the actual value of the maximum relative error is 19.7%, with an average error of 5.3%. The constitutive model can show a better description of sunflower seeds in the press process of plastic deformation. Kuhn's yield criterion can be used as theoretical basis for plasticity model of sunflower seeds. 