Based on measuring the reflection coefficient of viscoelastic layer and using the finite element method, the calculation method of viscoelastic dynamic mechanical parameters are developed. Measuring a viscoelastic layer specimen under two different acoustic backings will result in two different reflection coefficients, and using the finite element model to solve the reflection coefficient of viscoelastic layer, two nonlinear equations which are used to calculate viscoelastic parameters will be obtained. Using Newton iteration method to solve the two equations, the complex longitudinal and shear wave speeds of viscoelastic material will be obtained. Thus, the complex elastic modulus and complex Poisson’s ratio will be derived easily. The viscoelastic dynamic mechanical parameters, such as complex elastic modulus and complex Poisson’s ratio of polyurethane have been measured, and the effects of measurement errors on the viscoelastic dynamic mechanical parameters are discussed：The real parts of complex elastic modulus and complex Poisson’s ratio are mostly sensitive to the difference of two-hydrophone magnitude, and the loss factors of complex elastic modulus and complex Poisson’s ratio are highly related to the difference of two-hydrophone phase.