A flexible stick of a bow exercise device includes a base and a top cap, and a first and second flexible plates are connected between the base and the top cap. Two ends of the first flexible plate are respectively connected to the base and the top cap. A first end of the second flexible plate is connected to the base and a second end of the second flexible plate is movably inserted in a slot defined in the top cap. A gap is defined between the first and second flexible plates when the stick is not bent. The top cap can be removed from the two flexible plates and the second flexible plate can be replaced with another one with different flexibility.