In one embodiment, the present invention relates to a belt tensioning device for a hollow blind that comprises: a holder seat equipped with a lower belt gear on the top portion of the holder, the holder seat is in clearance fit with a lumen of a side frame of the hollow blind and is provided with a group of V-shaped slots from top to bottom therein, each V-shaped slot is equipped with a clip therein, the slip has a width slightly larger than the width of the lumen of the side frame of the hollow blind when being bent. The present invention allows the belt tensioning device to move along a single direction within the side frame of the hollow blind so that the distance between two belt gears can be gradually adjusted, thereby maintaining a proper tensioning force in the belt.