Abstract: Viscoelastic heat is one of the main heat sources during ultrasonic welding of thermoplastics.Unfortunately the present works could not definitely take into account the dynamic viscoelasticity of thermoplastics.In this paper a simplified method was proposed to characterize dynamic viscoelasticity based on static relaxation modulus and TTEP(time-temperature equivalent principle).The method could represent dynamic modulus as the function of temperature and frequency, and avoids calculating dynamic modulus of high frequency by shifting and extrapolating the dynamic modulus of lower frequency from experiments.With the method mentioned above, a strategy was put forward to simulate the viscoelastic heat of PMMA(polymethyl methacrylate) under periodic load using FEM(finite element method).The calculated temperature shows a similar tendency as tested results in other literatures.