By the quasi-static tensile,compression,and shear tests at room temperature with the loading rate 2 mm/min,the mechanical behavior and Poisson's ratio of polystyrene foam material were investigated.The results show that,the material has excellent ductility behavior on compression,but on tension and shearing,it is brittle with convergent Poisson's ratio of about 0.28.In addition,from the angle of engineering,based on the stress-strain relationship curves,its simple constitutive relation is given.By the constitutive relation a FE model to simulate an experiment is obtained,and the results show that the constitutive relation model is suitable.