The invention discloses an underground tunnel fabricated-type arch frame supporting system and a mechanical construction method. Primary spraying is conducted on an excavated tunnel, and an anchor rodis arranged in a struck mode; according to the arch frame design outline, arch frame trial assembling is conducted; all connecting joints of an arch frame are opened to conduct arch frame transporting; arch frame mechanical mounting is conducted through an arch frame auxiliary mounting machine; after the arch frame is accurately positioned, section reexamination is conducted through a laser distance measuring device, and an foot-locking anchor rod is arranged in a struck mode; and the rest of arch frames of the cycle are mounted till all the arch frames are mounted. According to the underground tunnel fabricated-type arch frame supporting system and the mechanical construction method, tunnel arch frame mechanical construction is achieved, the manual labor intensity is relieved, the construction work period is shortened, the construction speed is high, the number of needed workers is few on site, and the cost is greatly reduced.