The invention discloses a mounting tool for an engine rear oil seal support, and particularly relates to the field of engine processing equipment. The mounting tool comprises a guide rod, wherein oneend of the guide rod is provided with a handle, the circumferential outer wall of the other end of the guide rod is provided with external threads, a limiting plate is arranged on the circumferentialouter wall of the other end of the guide rod, the circumferential outer wall of the guide rod is sleeved with a mounting sleeve, a fixed sleeve is welded to one end of the mounting sleeve, a fixing hole is formed in the circumferential outer wall of the fixed sleeve, and a fixing pin is inserted into the fixing hole. According to the mounting tool, three arc-shaped top plates are arranged, and theengine rear oil seal support is sleeved on the outer walls of the three arc-shaped top plates; three connecting rods are opened outwards after being extruded by a top cylinder, so that according to the requirement for fixing the engine rear oil seal support with different inner diameters, the size of the surrounding formation formed by the three arc-shaped top plates is adjusted; and the mountingtool is suitable for installation requirements of the engine rear oil seal support with different inner diameters.