The invention relates to a rear registration plate lamp assembly of a motorcycle, which is provided with a lamp holder (1), a lamp shell (2) and a bulb (3), wherein the bulb (3) is arranged on the lamp holder (1); the lamp shell (2) is connected with the lamp holder (1); and a position on the lower lateral surface of the lamp shell (2) relative to the bulb (3) is provided with a transparent whitefilm (4). Besides, the lamp holder (1) is arranged in the lamp shell (2), a backlight surface on the lamp holder (1) and positioned below the bulb (3), and the surface of the transparent white film on the lamp shell (2) are inclined planes relative to a horizontal plane, a light reflecting surface on the lamp holder (1) and opposite to the bulb (3) is vertical to the horizontal plane, and the lower edge of the light reflecting surface is higher than the upper edge of an outer lateral surface of the transparent white film (4). The rear registration plate lamp assembly of the motorcycle makes arear registration plate lamp of the motorcycle more convenient for manufacturing on the premise of no exposed light according to national standards, and is helpful for improving the appearance commodity of the rear license plate lamp.