Thermal imbalance is usually encountered in the application of ground-source heat pump (GSHP) in cooling or heating load dominated area's, which results in the performance degradation of the system. Hybrid ground-source heat pump (HGSHP) assisted with cooling tower has been developed to attemper the underground heat accumulation in cooling load dominated areas. Based on TRNSYS 16, the performances of parallel and serial HGSHP systems are, respectively, studied in this paper. Considering the ground temperature increase and the total power consumption in the 20-years running time, 50% of the auxiliary cooling ratio (ACR) is found to be the optimum for both parallel and serial HGSHP systems. A control strategy which is named fixed load ratio (FLR) is proposed for the parallel HGSHP system. 0.5 of the load ratio is the optimal FLR, which is concluded from the 20 years' performance simulation. The fixed entering temperature (FET) control strategy is studied for the serial HGSHP system, and the optimal temperatures that cooling water enters the heat pump to start up and shut down the cooling tower are 30 degrees C and 28 degrees C, respectively. (C) 2015 Elsevier B.V. All rights reserved.