The present invention relates to an assembling device of built-in U-shaped permanent magnet motor permanent magnet and an assembling method thereof. The assembling device comprises a fixing portion and a feed portion. The fixing portion comprises a bottom plate, a workbench, a linear guide rail and a fixing end plate; the workbench is vertically fixed at the upper end of a horizontal bottom plate,the lower end of the linear guide rail is connected with the workbench, and the upper end of the linear guide rail is connected with the fixing end plate; the feed portion comprises a motor, a screwrod, a permanent magnet guidance groove, a pneumatic device and a permanent magnet propulsion guidance block; the motor is fixed at the upper end of the fixing end plate, the lower end of the motor isprovided with the screw rod, the screw rod is connected with the permanent magnet guidance groove, the motor and the screw rod are rotated to drive the permanent magnet guidance groove to slide up and down along the linear guide rail, the pneumatic device is fixed at the upper end of the fixing end plate, the pneumatic device is connected with the permanent magnet propulsion guidance block through a push rod to drive the permanent magnet propulsion guidance block to slide up and down along the linear guide rail. The problem is solved that the assembling of the built-in U-shaped permanent magnet motor permanent magnet is difficult.