<p>Numerical simulations of overflow gas-assisted co-injection molding (OGACIM) were conducted using self-developed UDF programs, and experimental investigations were performed on a self-built OGACIM platform. The synergistic regulation mechanism involving geometric, rheological and the thermal fields in the OGACIM process was systematically revealed. Simulations indicated that circular cavities exhibited the best fluid uniformity and temperature stability, followed by square ones. Non-circular sections developed local high-temperature zones due to the shear effect of outward convex right angles, while rectangular sections exhibited significant heat loss because of the largest surface-area-to-volume ratio. Based on the simulation study, OGACIM experiments were conducted on five cross-sectional cavities: circular, semicircle, square, rectangular, and “convex”. The residual wall thickness evolution is jointly controlled by the gas–melt interface temperature field and rheological behavior, both influenced by cross-sectional geometry. The maximum total residual wall thickness of the OGACIM specimen increased as a whole and the fluctuation intensifies with the decrease of the circle rate, while the fluctuation range of the minimum total residual wall thickness narrowed. Its evolution is synergistically governed by the temperature field at the gas–melt interface and the rheological behavior, both of which are shaped by the cross-sectional geometry. The circular cavity had the smallest fluctuation range in inner melt penetration rate, while the “convex” shape had the largest. These results provide a theoretical foundation and technical support for optimizing product cross-sectional design and process parameter adjustment in gas-assisted co-injection molding, thereby improving product wall thickness uniformity and quality stability.</p> Graphical abstract <p></p>