In injection molding processes, while the plastic is injected into the mold cavity, the displaced air leaves the mold through openings which are referred to as the venting slots. With the continuous improvements of manufacturing precision and injecting speed, the design of venting slot is becoming an important problem which influences the quality of products greatly. In this paper, five types of possible location to generate “gas-trap” were summarized based on the research and analysis of mold manufacturing and injection molding. The dimensions of venting slot, eight kinds of venting mode and their corresponding structures were presented. The principle for the design of venting system was proposed.