An apparatus for pre-loading a mechanical constant-resistance single prop includes a rhombic stretchable bracket, a screw rod, a first cylindrical sleeve ring, and a second cylindrical sleeve ring. The screw rod is connected to two opposite apex corners of the rhombic stretchable bracket. A tail end of the screw rod is fixedly connected to one apex corner of the rhombic stretchable bracket. An end head of the screw rod passes through another apex corner of the rhombic stretchable bracket. The remaining two opposite apex corners of the rhombic stretchable bracket are respectively fixedly connected to edges of the first cylindrical sleeve ring and the second cylindrical sleeve ring through connecting rods. The screw rod is threadedly connected to the apex corner of the rhombic stretchable bracket through which the screw rod passes. A method for pre-loading a mechanical constant-resistance single prop is also provided.