The invention belongs to the technical field of bathroom accessories, particularly relates to a lower seat of a combined rotary shaft seat, and solves the technical problems of poor stability and the like in the prior art. The lower seat of the combined rotary shaft seat comprises a seat body, wherein the lower end of the seat body is provided with a bottom case, the upper end of the seat body is provided with an opening, a dividing structure capable of dividing a chamber into a positioning sub chamber and a round sub chamber is arranged between the inner wall of the seat body and the upper bottom face of the bottom case, the bottom case is provided with at least one positioning upright post located in the positioning sub chamber, one end of the positioning upright post is fixedly connected with the bottom case, the other end of the positioning upright post extends to the outside of the opening end of the seat body, a positioning hole is formed in the axial center of the positioning upright post, the bottom case is also provided with a rotary shaft positioning post penetrating through the axial center of the round sub chamber, one end of the rotary shaft positioning post is fixedly connected with the bottom case, and the other end of the rotary positioning post extends to the outside of the opening end of the seat body and is flush with the end of the positioning upright post. The lower seat of the combined rotary shaft seat has the advantages that the connection stability can be improved, and a door is convenient to dismount.