On the basis of analyzing the common method to determine the Poisson’s ratio and based on the generalized Hooke’s law, deduced a calculation formula of Poisson’s ratio under the triaxial stress state. According to the existing ratio method, tangent method and derived formula, compared and analyzed the Poisson’s ratio variation of the rubber-sand mixture under triaxial tests. The results show that compared with the tangent method and ratio method, the change rule of Poisson’s ratio of rubber sand obtained by the derived formula is more reasonable. With the increase of the content of rubber particle, Poisson’s ratio of rubber-sand mixture displays a decreasing trend. Under the condition of small confining pressure, the confining pressure has little influence on Poisson’s ratio.