When the heat insulation layer of ship thermodynamic pipeline is damped, its heat insulation effectiveness can be reduced and the pipeline can be corroded. A model of damped heat insulation layer is established. The finite volume method is used to calculate the variation of temperature field, outer surface temperature and heat transfer loss of the heat insulation layer of a vapor pipeline with the damp thickness of the layer. The result shows that when the damp thickness is up to 20 mm, the temperature of the heat insulation layer will exceed its standard. For a damp area with same thickness, the nearer the outer surface is, the higher its temperature is. This reveals the importance of protecting the outer surface of a heat insulation layer and provides the basis for infrared thermal imaging diagnosis of heat insulation layers.