The invention relates to a high-precision installation device for a frameless motor, which comprises a guide shaft (1), a bottom plate (2), a bracket (3) and an upper cover plate (4), and is characterized in that the bracket (3) is installed on the bottom plate (2), and one side of the guide shaft (1) is fixed in an installation groove of the bracket (3) through the upper cover plate (4); and theguide shaft (1) serves as a guide rail of a motor shaft (5), and when the frameless motor is installed, the motor shaft (5) is arranged on the guide shaft (1) in a sleeved manner, moves along the guide shaft (1) and fixes relative positions of a motor stator (7) and a motor rotor (9) in the axial direction and the radial direction through a harmonic reducer (10), a bearing (11) and a bearing frame(8) when moving to the installation position. Through using the device and method, the clearance of the frameless motor rotor (9) and the motor stator (7) in installation can be ensured, the motor rotor (9) and the motor stator (7) are not attracted together due to excessive magnetic field attraction, and the coaxiality of the motor is also greatly improved.