Upon summarization and analysis of the structural features of automobile large plastic parts and the selection of corresponding shaping modes, this paper analyzes and elaborates product design, raw material selection, forming method, structural design of mode, and debugging of shaping process, etc. on account of manufacturing defects such as weld line and sink mark, etc. easily occurring to automobile large-scale plastic decorating parts subject to injection molding, and offers operable solutions and measures available for practical production for the sake of the maximum improvement of product apparent quality.