The work aims to prepare functional wood-plastic composite by renewable pulp fiber and functional plastic polycarbonate (PC) and investigate the effects of boric acid treatment on the properties of the composite to further expand the application of composite in the field of engineering. 5% (mass fraction) boric acid solution was prepared, and part of the pulp fibers were immersed in boric acid solution for 2 h. After mixed with PC, the pulp fibers were fused by a double spiral extruder and then subject to injection molding by a miniature injection molding machine to prepare the composite. The elastic modulus of the composite treated with boric acid reached 4.31 GPa when 30% pulp fiber was added, which was 16% higher than that of the composite without boric acid treatment but with the same fiber content. At the same time, compared with pure PC, the elastic modulus increased by 90%. The thermal stability of pulp fiber is improved by boric acid treatment, and the mechanical strength of the composite is further enhanced. PC is a common functional plastic in life, which has excellent mechanical properties. The treatment by boric acid and the addition of pulp fiber further improve the mechanical properties of the composite, which is expected to be used as a structural material for containers, pallets, etc., and has a good prospect in the field of packaging application.