H13 steel has often been used as a hot-work die material. However, it tends to undergo failure because of wear and crack generation under high-temperature and high-pressure working conditions. Laser cladding is an effective remanufacturing method for such materials. However, the temperature gradient and cooling rate of the formation process are very large owing to rapid cooling and heating characteristics, often inducing excess thermal stress and cracking of the coating. Herein, a numerical simulation of a laser cladding 316L/H13+20%WC composite coating on H13 steel surface was performed. The variation in the temperature gradient and cooling rate with time and the influence of substrate preheating on the temperature gradient and cooling rate were studied. Moreover, to verify the numerical simulation results, a laser cladding formation test of the composite coating was conducted on the H13 steel substrate. Experimental results show that substrate preheating can significantly reduce the temperature gradient and cooling rate on the top surface of the coating and exerts a certain restraining effect on the cracks of the sample surface. The findings of this study provide a reference for the laser cladding modification and repair of H13 steel dies.