To test the cooling performance of a Fan Coil Unit (FCU), a large amount of experiment has to be carried out, which is both time-consuming and energy-consuming. To solve this problem, in this paper, the grey system theory is used to establish a GM (1, N) grey model representing the relationship between the FCU volume and such parameters as the frozen water temperature, the cooling water volume and the wind temperature, to predict the FCU cooling performance under various working conditions. The results show that both the simulation and the estimation are satisfactory, and the method provides, a scientific and effective way for FCU enterprises and testing departments to estimate the working condition characteristics of new batches of FCU with only a few (5 to 6) typical experimental data and with the historical data of working conditions, thus to reduce the test time and the energy consumption to less than 5% of those by using the traditional method, and to enhance the FCU characteristics testing work efficiency by 20 times.