The temperature field and the relation between surface temperature of the insulation layer and its damped thickness are calculated by finite element method. The results indicate that for a heat insulation material the surface temperature and heat losing of pipeline will increase with its damped insulation layer thickness increase ,and it will exceed permissibility when the thickness reaches its 50 %. At last ,the temperature field of ship steam pipeline caused by mangled heat insulation layer and the relations between the surface temperature increase and the flawed thickness are calculated ,and those are validated by infrared imaging picture. The gist for infrared diagnosis of heat insulation layer is offered.