Carbon fiber paper (CFP) has been widely investigated as a gas diffusion layer for high-performance fuel cells, while the paper formation, one of the most important properties of CFP, has been rarely reported before owing to the lack of accurate characterization methods. Herein, using a 2D laboratory sensor, a quantitative analysis was conducted to comprehensively analyze CFP formation, carbon fiber (CF) flocs and their influencing factors. Within the study scope of this paper, improved paper formation was always accompanied with a smaller floc area and a lower ratio of large-size flocs for CFP prepared with the same length of CF. CF was too long negatively affected paper formation, as the look-through index rose from 282 (2 mm CF) to 940 (8 mm CF). Either using polyethylene oxide as a disperser or reducing the CF concentration could reduce the look-through index of the CFP (2 mm CF) to below 200. The crowding factor of the CF suspension was suggested to be no more than 5. In addition, improvement in CFP formation led to higher tensile strength and lower air permeability in CFP. All these results are expected to prepare CFP with superior formation, thereby improving other properties of CFP.