Abstract: in the concept and meaning of elastic and viscoelastic Poisson's ratio are analyzed. The integral equation between the tensile stress relaxation modulus E(t),volume relaxation modulus K(t) and viscoelastic poisson's ratio v(t) are derived. A method of calculation for the numeric integral for solving v(t) are obtained. Based on experimental results of E(t) and K(t), a law for their variation and formula on viscoelastic poisson's ratio, taking a modified double propellant as a real example are given.