By analyzing the composition of the direct incremental cost and the incremental cost in the full life cycle of water-saving projects for green building,an incremental economy model,which includes both the initial and the future cost in the full life cycle,is established in the form of net present-value measurement index,and a method to calculate the direct incremental cost is proposed.Then,the model is simulated and the method is applied to a water-saving demonstration project for green building in West China.The results show that the direct incremental costs per unit construction area in the full life cycle are respectively 21.0 yuan per square meter and 31.5 yuan per square meter,which means that the proposed model and method are both reasonable and practical.