The invention discloses a large electric motor rotor casing tool and casing method which can help solve a problem that hidden danger of large accidents exists in conventional large motor rotor casingprocesses. The large electric motor rotor casing method comprises the following steps: a stator (3) lies horizontally on a stator support rack (2); a rotor carrying trolley (27) and a rotor follow-uptrolley (5) and a follow-up support rack (6) are respectively arranged on a guide rail (4) in movable manner; a rotor lies horizontally on the rotor carrying trolley (27), a driving electric motor (13) drives a lead screw nut (26) to rotate via a belt, and a lead screw (25) above a cantilever beam is driven to perform axial linear motion; as the lead screw (25) is arranged between a left top connection plate (23) and a right top connection plate (24) disposed on a top surface of an assembly guiding sleeve cylinder (10), the lead screw (25) is axially linearly moved to drive the assembly guiding sleeve cylinder (10) to move, and therefore the rotor (7) is cased in the stator (3). The large electric motor rotor casing tool and casing method disclosed in the invention can help overcome a problem that a head of the cantilever beam can easily fall down during rotor casing processes, and the rotor can be cased in a stable and safe way.