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Thermal expansion process is widely used for manufacturing carbon fiber composite products with box, tube shape and multi-cavity structure. The compacting pressure on prepreg stack is generated by core mold with high thermal expansion coefficient inside a closed mold, which is balanced by resin pressure inside prepreg stack and the effective stress of fiber bed. Resin pressure is one of the most important parameters for controlling the quality of composite, which significantly influences resin flow and defect formation. In this study, resin pressures inside a carbon fiber composite joint during thermal expansion process with R10301 silicone rubber core mold and Aircast 3700 silicone rubber core mold were measured by means of a self-developed resin pressure online monitoring system. And resin flow at different positions of the composite joint was analyzed according to the resin pressure variation. Besides, fiber volume fraction and porosity of different positions were investigated through micrographs to evaluate the quality of the joint. The experimental results demonstrate that the online monitoring system can well meet the requirement for measuring resin pressure inside the prepreg before gelation. The resin pressure profiles of the composite joints processed with R10301 silicone rubber core mold and Aircast 3700 silicone rubber core mold are significantly different, which are greatly influenced by thermal expansion coefficient, bulk elastic modulus of the silicone rubber core mold and the process gap. On the other hand, the resin pressure at the corner area is much lower than those at other areas, resulting in voids and resin-rich defects. These results provide a basic understanding on the resin pressure characteristic inside the composite joint during thermal expansion process and a direct guidance for process optimization to improve the quality of composite product.