Fiber reinforced composites have been an important way to utilize agriculture plant fibers. Flax fiber is plant fiber and strong, biodegradable, anti-fungi and bacterial. Flax fiber reinforced PP fiberboard is thermoplastic with the advantages of low density, low cost, low energy consumption, and recyclable. The influences of flax / PP fiber blending ratio, molding temperature, molding time on the mechanical properties of flax / PP board were carefully investigated. After mathematical manipulations and experimental validation, it was found that the thermoplastic composite board had maximum tensile and bending strengths when the flax fibers were at 50% of the total weight, the molding temperature was 181 ， and the molding time was 48 minutes.