The utility model relates to a conveyor belt tensioning device, in particular to a constant force tensioning device of a V-shaped bottom of a mine belt conveyor. According to the technical scheme, the constant force tensioning device comprises a conveyor belt device, a conveyor belt tensioning device, a counter weight suspension device, a tension trolley and a steel wire rope steering pulley block. The conveyor belt tensioning device is arranged at the V-shaped bottom of the conveyor belt device and comprises a tensioning support and a first steering cylinder. Sliding tracks are arranged on the two inner sides of the tensioning support, the first steering cylinder is arranged at the conveyor belt forwarding end of the tensioning support, and the first steering cylinder is fixed by a directional bracket independent of the tensioning support. The tensioning trolley is pulled to a conveyor belt to be tensioned when a counter weight is in a free state, and the looseness problem of the conveyor belt is solved. Moreover, elastic stretching of the conveyor belt can be counteracted and the conveyor belt can be stretched permanently.