The invention discloses a mechanical plate shearing machine adopting three-point supporting and a machining method thereof, and belongs to the field of plate shearing machines. According to the mechanical plate shearing machine adopting three-point supporting, protruding sections are arranged at the lower end of a front face plate, roller shaft holes are machined in wall plates and the front face plate, and the three-point supporting structure is applied to the mechanical plate shearing machine. For the plate shearing machine, the main machining method includes the steps that the roller shaft holes in the wall plates and the front face plate are machined first; then, a main shaft and a drive shaft are installed; then, the front face plate and a bracket are welded and installed, an installed machine frame is subjected to overall machining, and the dimensional precision of the roller shaft holes is ensured; and finally, machining and installing of other parts are conducted. The mechanical plate shearing machine and the machining method solve the problem that abrasion to a traditional mechanical plate shearing machine is large, the dimensional precision of the machine frame is improved by controlling the machining sequence and process, the product machining quality of the plate shearing machine is easily improved, and the service life of the plate shearing machine is easily prolonged.