The invention relates to a conveying belt tensioning device, in particular to a V-shaped bottom constant force tensioning device for a belt conveyor of a mine. According to the technical scheme, the V-shaped bottom constant force tensioning device comprises a conveying belt device, a conveying belt tensioning device, a heavy hammer suspension device, a set of tensioning trolley and a steel wire steering pulley block. According to the V-shaped bottom constant force tensioning device, the tensioning trolley is pulled till a conveying belt is tensioned by a heavy hammer at a free state, the problem of loosening of the conveying belt is solved, and the elastic elongation and the permanent elongation of the conveying belt can be absorbed.