The invention relates to carbon fiber paper and a preparation method thereof, in particular to carbon fiber paper which has low cost and high structural stability and can be used for gas diffusion layers of proton exchange membrane fuel cells (PEMFC) and a preparation method thereof. The carbon fiber paper of the invention comprises PAN-based graphite fibers and resin carbon and is in a three-dimensional reticular structure formed by the PAN-based graphite fibers bonded by the resin carbon, the resin carbon is tightly lapped on the junction of every two PAN-based graphite fibers, and the mass percent of the resin carbon of the carbon fiber paper is 10-20%. The preparation method of the carbon fiber paper of the invention comprises the steps of activating, short cutting, paper making and molding by a wet process, resin spraying, drying, hot pressing, heat treating and the like. The invention solves the problems of time consumption and energy consumption of activating treatment of the carbon fiber paper in the prior art, overhigh cost of the carbon fiber paper, unsatisfactory bonding effect of the traditional phenolic resin and unstable structure of the carbon fiber paper after high-temperature heat treatment, and overcomes the defects of the prior art.