The invention discloses a special surface treating agent for PBO fibre, which is characterized by comprising 15-30% of terminal carboxyl group-containing liquid rubber with the molecular weight of 2500-3500 and 70-85% of solvent acetone according to the mass percent. The invention also discloses a method for preparing a composite material by the PBO fibre processed by the special surface treatingagent. The method comprises the following steps: immersing the PBO fibre into a container provided with the special surface treating agent, immersing the processed PBO fibre into homogeneous phase basal body glue liquid, then winding PBO fibre bundles on an NOL ring mould and solidifying at room temperature; then, putting into a vacuum drying oven, heating, solidifying, cooling naturally to the room temperature, demolding to obtain a PBO fibre reinforcement composite material. The shearing strength value of the composite material prepared by the PBO fibre processed by adopting the special surface treating agent is enhanced from 12-25MPa of the prior art to 26.28-29.32MPa.