The temperature distribution in optical coating was investigated. An idea about temperature field design of optical thin film is presented here. The peak temperature can be reduced and the damage threshold is improved by advanced coating design. The thermal process in optical coating irradiated by laser pulse was investigated with photothermal deflection technique. It was confirmed that the photothermal deflection technique is a very useful method for in-situ research of temperature process and damage process induced by laser pulse. Some experimental results of laser induced damage in optical coating, such as the spotisize effect, accumulation effect, effect of absorption on damage threshold, coating thickness effect, and so on, are presented in this paper.