Products made from double-layered hollow vacuum forming are widely used for their various advantages. The hollow vacuum forming process has been studied in this paper. Numerical simulation method for the hollow vacuum forming process of double-layered plastic sheets has been introduced by the simulation software Polyflow, which is suitable for viscoelasticity fluid bodies. This method can vividly and intuitively estimate the thickness, temperature variation and distribution in the double-layered vacuum forming processes. Based on this method, reliably theoretical calculation data can be provided to design the reasonable vacuum forming process for double-layered vacuum forming of new materials or new products. The proposed method has been verified to be applicable and effective by prototype fabrications.