Abstract: The formation process and causes of the skin wrinkles under the radius filler were studied in the co-bonding process of the hat-stiffened skins of carbon fiber/epoxy resin composite with the pre-cured stiffener and uncured skin. The rheological properties of the resin of the prepreg were tested. The internal pressure distribution of the skin under the radius filler was monitored by pressure monitoring equipment, and the wrinkles were characterized by metallographic microscope. The results show that during the window period of the resin flow, the resin flows to the low-pressure area and aggregates, resulting in wrinkles when a pressure difference occurs inside the skin under the radius filler.