The experiment of laser alloying on the surface of P20 steel was made. Tungsten carbide (WC) powder was used as alloying material. The microstructure and property of laser alloying layer and nitriding layer are studied. The research showed that laser alloying layer had better properties such as minute crystals, deeper layer, higher hardness and good metallurgical bonding with base metal. The average hardness of alloying zone was 600HV0.2. The average hardness of phase-change hard zone was 450HV0.2. P20 steel was widely used in the field of plastic mold manufacture, especially mold core and cavity. Abrasion, corrosion and pressure resulted in change of mold size and shape which could greatly affect molding precision. Using laser alloying, the good wear layer would be made on the surface of p20 steel and would greatly increase the mold useful life.