A counterweight device is mounted to a hitting trainer. The hitting trainer includes a base, an upright rod disposed on the base, and a hitting member connected to the upright rod. The counterweight device includes a counterweight bag which can be extended or compressed. When in use, the counterweight bag is fitted on the base and then the counterweight bag is pulled to unfold. Finally, external liquid is poured into the counterweight bag to increase the weight of the counterweight bag, such that the hitting trainer can be placed on the uneven ground stably. When the counterweight device is not used, the counterweight bag is detached from the hitting trainer and compressed to reduce the volume of the counterweight bag for storage and carrying.