The invention relates to a speed reducer used for an automobile tail door supporting rod. The speed reducer comprises a base. A primary planetary reduction mechanism and a secondary planetary reduction mechanism which are coaxial are arranged on the base. The primary planetary reduction mechanism comprises a primary planetary reduction frame which is provided with a primary sun gear and multiple primary planetary gears, wherein the outer rings of the primary planetary gears are engaged with a primary outer gear ring. The secondary planetary reduction mechanism comprises a secondary planetary reduction frame, a secondary sun gear is arranged above the primary planetary reduction frame, and the outer ring of secondary planetary gears are engaged with a secondary outer gear ring. The primaryouter gear ring is connected with the secondary outer gear ring through a buckle. The speed reducer is simple in structure, compact in design and convenient to use, helical teeth are adopted in the primary planetary reduction mechanism and the secondary planetary reduction mechanism, the reduction ratios of the primary planetary reduction mechanism and the secondary planetary reduction mechanism are the same a shell and the gear rings are combined to be of an integrated structure which is simpler, and in addition, the primary outer gear ring and the secondary outer gear ring are connected andinstalled conveniently through the buckle.