The insufficient filling and shrinkage are often produced in the process of injection molding. These defects cause the production efficiency to reduce and production cost to rise. In this paper, the forming process of Dn40-type control valve is analyzed to find the main reason for such forming defects by using CAE tools and comparing with the good forming process of a Dn50-type control valve. Results show that big difference between wall thicknesses and bad exhaust system in injection mold is the main reasons for insufficient filling. In the meantime, measures to prevent defects are investigated. Modifying the local structure of the injection mold is provided to avoid the defects. Using reasonable parameters in the injecting process and modified injection mold, the injection product have a smooth surface and good forming. The experiment verified the feasibility of the measure for prevention of defects.