Molding is an important polymer processing technic, and the molding shrinkage of products is the key factor of the precision and qualified rate of polymer molding products. Based on the visualization molding system, activated carbon particles as feature points were added into the polymer material, and a visualization method for measuring the inner molding shrinkage of plastic molding products was put forward. The method was applied to measure the inner shrinkage of polyurethane elastomer products. The results show that this method is effective for the measurement of internal shrinkage. According to the measurement results, there is obvious negative internal shrinkage near the gate, and the inner shrinkage is related to the flowing behavior of the polymer within the mold cavity.