The embodiment of the invention provides an installation frame of a purely-mechanical overspeed protection device and an installation method, and relates to the field of mechanical design. The installation frame comprises an installation supporting arm and an installation clearance adjusting device. The installation supporting arm comprises a first sliding groove, a sliding installation plate and a supporting arm flange. The installation clearance adjusting device comprises a rolling threaded rod, a sliding rod, a sliding rod flange, a sliding frame and a dial indicator. The threaded rod is rotated to control a release device to be installed to slide along the first sliding groove so as to adjust the installation clearance, the stroke of the threaded rod is accurately measured through the sliding frame and the dial indicator so as to determine the installation clearance between the release device and a centrifugal detector, and therefore the operation is convenient, and the error is small.