Experiments on the long-term behavior of practical U-shaped ground coupled heat exchangers (GCHE) were carried out. Based on the grey theory, GM(1,1) model was established, and further topological prediction was given for key parameters including heat-exchange rate Q and the coefficient of heat transfer K. Simulated results showed that there is a better coincidence between the test and predicted data. During the long-term operation of the GCHE, due to the unbalance of the load heat transfer is likely to become worse and worse, especially in the summer. The summer mode can be divided into three working zones: transitional period, steady period and weakening period. A dimensionless runtime τ is presented to express the long-term behavior of K, where K = aτ^b. The runtime τ of three working zones is determined as 0-25%, 25-80% and 80-100%, respectively. Prey prediction can provide us with a quantitative basis for stable operation and next optimization of the ground source heat pump system (GSHP).