With gray system theory as the basis and the Excel software as the implementation tool, put forward a Excel algorithm based on gray relational analysis for the optimum selection of air conditioning heating and cooling source system. Introduced the principle of gray correlation analysis method and the function needed to use in Excel software. Through an engineering example, described the process of using gray relational analysis method to select the optimal plan from five air conditioning heating and cooling source systems by Excel software. The results show that, the correlation degree sorting between the schemes of the five heating and cold source systems and the reference scheme is R(1)>R(3)>R(2)>R(4)>R(5) and the calculated results agree with the actual, which indicates that the method is scientific, simple and practical. Provides reference for different levels of HVAC engineering.