Currently, the fluid exchange among CO2, brine and freshwater is rarely reported, associated to CO2 leakage along activated fault during geologic storage. To investigate the fluid exchange, a 3-D numerical model was proposed considering CO2 leakage along two faults. Through analyzing the leakage rates of CO2, brine and freshwater, the fluid exchange associated to CO2 leakage was observed among CO2, brine and freshwater. Then, the exchange mechanism was concluded, including the major fluid exchange between two faults, as well as the minor fluid exchange along the same one fault. In the major fluid exchange, CO2 leaks from saline aquifer to freshwater aquifer along fault near the injection well, while freshwater leaks from freshwater aquifer to saline aquifer along fault far from the injection well. The major fluid exchange is harmful for CO2 geological storage, because more CO2 leaks upward into freshwater aquifer in the fluid exchange. In the fault near injection well, the minor fluid exchange is observed, i.e., CO2 leaks upward and freshwater leaks downward. In the fault far from injection well, the minor fluid exchange is also found, i.e., brine leaks upward and freshwater leaks downward. An implication from the fluid exchange was obtained through setting pump wells. The results show that setting pump wells reduces the leakage amount of CO2 and brine obviously. Additionally, based on the numerical model, the parametric analysis was performed on the fluid exchange, including the permeability of fault, the permeability of saline aquifer and the fault width. Especially, the fluid exchange was herein reflected by leakage rates and amounts of CO2 and freshwater. The results show that: (1) CO2 and freshwater have larger leakage rates and amounts when increasing the permeability of the fault and the saline aquifer; (2) the width of the fault far from injection well has a more significant effect on the leakage rates and amounts of CO2 and freshwater, compared to the width of the fault near injection well. (C) 2018 Elsevier Ltd. All rights reserved.