Abstract: For the carbon fiber satin fabric/epoxy 914 prepreg, a resin pressure online measuring system for hot pressing process was used to measure the value and distribution of resin pressure. Additionally, the effects of vacuum and autoclave pressure on resin pressure were studied. Moreover, the influence of vacuum and autoclave pressure on void formation was investigated using micrographs. The experimental results show that the online measuring system applied can quantitatively analyze the influence of vacuum in plies and resin pressure evolution. The resin bears most of autoclave pressure and distributes uniformly in thickness and in-plane directions during autoclave zero-bleeding process. Vacuum can exhaust entrapped air through gas channels in prepreg stacks, and the effect depends on resin rheological property and compaction of prepreg stacks. In addition, the results of void analysis are consistent with the measurements of resin pressure under different pressure conditions.