In this paper, the forward hot diaphragm machine with single film and the plate lamination of thermosetting composite manufactured by automated tape laying technology were setted as the research object, and the relevant parameters and quality influencing factors of C-shaped single diaphragm preforming, such as lay-up sequence, preforming temperature, and vacuum pressure rate were studied respectively. Numerical simulation methods were used to verify the results. The quality of hot diaphragm preforming is mainly determined by the layer design, and the optimization of process parameters can only improve the occurrence of defects. And the combination forms of process parameters varies for different types of composite materials, preforming tool structure, and component structure. The rules summarized in this paper have been successfully applied on the preforming process of composite components of high aspect ratio with different sizes, thicknesses, and shapes, providing support for the automated manufacturing of complex composite main load-bearing components with large aspect ratios.