Abstract: In refineries, insulating layers are often applied to pipes in order to reduce energy loss and meet the process requirements. Definition of economic thickness of insulating materials and insulating layer is essential for reducing the pipe operation costs. In this paper, an optimization model of insulating layer for high-temperature overhead pipes in refineries is established, which considers insulating layer thickness as decision variable, outside surface temperature and insulating efficiency as constraints, and minimum annual pipe expenses as target. Taking the pipe in a refinery in South China as an example, its insulation optimization is discussed, showing that the current rock wool is still a kind of insulating materials with the lowest sum of heat loss cost and investment expense, but the economic thickness defined is 120 mm. The relations between insulating layer thickness and heat loss at different outside surface temperatures and between insulating layer thickness and insulating efficiency for different materials are identified, and calculation is made by setting proper maximum surface temperature and insulating efficiency depending on actual pipe operations and types of insulating materials. Accordingly, the optimum insulating layer is obtained.