Recently, with the expansion of application of polymer composite materials, high levels of deformation compensation actions have been developed. However, there is a problem of high-temperature viscoelasticity that occurs over time after completing the injection molding process. In this study, changes of mechanical properties of the Moldflow program for injection molding were analyzed to verify the viscoelasticity phenomenon through deformation analysis. In addition, deformation analysis of plastic injection molded products according to arrangement of three ribs was conducted and two products with different geometric shapes of the same function were compared. As a result, it was possible to reflect the viscoelastic effect by reducing the elastic modulus and shear modulus of the material. It was confirmed that the geometric shape with thick ribs formed in multiple longitudinal directions was mainly responsible. On the surface of the product where the rib arrangement was parallel and perpendicular to the flow direction, the orientation was orthogonal to the linear direction and the maximum residual stress was 81.17 MPa, which showed the largest value. It was judged that viscoelastic phenomena could be predicted and that an arrangement of parallel and perpendicular ribs that might intersect should be avoided.