The suction plastic forming process for in-mold decoration plastic sheet has been the best process for thin-shell plastic exterior decoration parts. But the suction plastic forming products still suffers from the uneven thickness. Based on the general finite element analysis software POLYFLOW for viscoelastic fluid, a set of optimization methods for suction plastic forming process of in-mold decoration plastic sheet is introduced in this paper to reduce the uneven level of thickness. These methods include establishing process optimization scheme, building mesh model, selecting the material constitutive model and determining its parameters, imposing boundary conditions and blowing pressure, and applying the mold movement. Finally, the optimized suction plastic forming process is used to produce the in-mold decoration plastic rear bumper sample of an automobile, and the results show that optimized process is effective and applicable.