The invention relates to the technical field of fire-fighting mounting boxes, and discloses a fire hydrant box door piece mounting mechanism which comprises a box body and a mounting frame, four edgefold strips arranged in the front face of the box body form a frame body, two cutting grooves are formed in the front faces of the edgefold strips, connecting strips are arranged in the cutting grooves, and round grooves are formed in the connecting strips; and the frame body is in lap joint with one side of the mounting frame, and one side of each connecting strip is in lap joint with the outer side of the mounting frame. When a door piece of a fire hydrant box is installed, installation is convenient, it is only required to fold up the connecting strips from the cutting grooves and enabling the mounting frame to be in lap joint with one side of each connecting strip, and the eight connecting strips in all are in lap joint with the outer side of the installation frame. Therefore, the connecting strips are fixed in the outer side of the mounting frame through the screws, the mounting of the mounting frame can be quickly realized, the mounting space is large, operation in the box body isnot needed, the operation efficiency is improved, the time is saved, and meanwhile, operation of an operator is facilitated.