Thermal stress caused by oversized temperature gradient in the process of laser cladding ceramic coating on metal substrate usually makes the coating cracked. Reducing the temperature gradient of the laser cladding process can reduce the thermal stress to a certain degree to suppress the generation of cracks. Through numerical simulation of temperature distribution during laser cladding Al2O3 coating on Ti6Al4V substrate, a method of providing preheating and slow-cooling using top hat laser beam is proposed. Temperature distribution and temperature gradient of the coating with preheating and slow-cooling are discussed by changing the spot size and power density of the top hat assistant laser beam. The results show that there is a significant feature of preheating and slow-cooling during the cladding process, the top hat assistant laser beam can reduce the temperature gradient effectively. The temperature gradient of the plastic point (1533 K) of Al2O3 reduces while the preheating and slow-cooling temperature raises. But the temperature gradient of the plastic point will rebound when the preheating and slow-cooling temperature is near the plastic point. Higher preheating and slow-cooling temperature make the cladding process better, but temperature around the plastic point should be avoided.