C-shaped beam sandwich structures with large length-to-diameter ratio were studied. Imported T 300 grade carbon fiber fabric prepreg was used to study the influencing factors of porosity of C-shaped beam sandwich structures, excluding the influence of curing system. The research mainly focused on the two processes of laying and packaging, and studied from the three influencing factors of pre-compaction parameters, auxiliary materials layout and air guide wire layout spacing. After setting different process parameters and testing the porosity of the manufactured C-shaped beam sandwich structure parts, the maximum porosity of the best process parameters is greatly reduced from 2.1% to 0.3%, which thereby improves the qualification rate of the parts.