Life cycle green cost assessment (LCGCA) method, which can evaluate building environmental load and economic performance throughout its life cycle comprehensively, is propounded in this paper in order to guide green building design. In LCGCA, environmental load (EL) cost is proposed based on concept of environmental tax and counted into general building initial cost and operating cost, and then green payback time (GPT) could be worked out. With this method, an office building in Beijing is studied. The operating energy consumption, life cycle EL, life cycle cost (LCC) and GPT of different envelope schemes have been compared. The results indicate GPT is obviously shorter than the general payback time when EL cost is considered. Especially, the exterior shading scheme changes to be economically feasible through GPT evaluation. This assessment method is more suitable to guide green building design practice because environmental performance and economical performance are considered together.