The invention discloses a method for improving performance of a composite material for a bipolar plate. A method for simultaneously improving the conductivity and the bending strength of the composite material for the bipolar plate is characterized in that grading is carried out on two kinds of natural flake graphite with different sizes; the particle size of large-size graphite is D; the particle size d of small-size graphite is 0.092D to 0.586D; grain grading is carried out on the large-size graphite and the small-size graphite at a reasonable ratio; the large-size natural flake graphite accounts for 60%-90% of weight percentage of the total weight; the small-size natural flake graphite accounts for 0-10% of weight percentage of the total weight; a proper amount of nano filler conductive carbon black (0-10%) or graphene (0-2%) are added to be compounded with a phenolic epoxy vinyl resin binder to prepare a molding compound; and the molding compound is molded and cured at certain temperature, pressure and time to prepare the sample. Through application of a grain grading and compounding technology, a relatively perfect conductive network is built by the composite material and a relatively good resin infiltration condition is obtained, so that the conductivity and the bending strength are simultaneously improved.