The invention relates to an automotive rear seat armrest cup holder structure with a variable inner diameter. The automotive rear seat armrest cup holder structure comprises a cup holder body, and thecup holder body is arranged on an armrest body. The automotive rear seat armrest cup holder structure is characterized in that a cup placing slot is formed in the cup holder body, and a pair limitingguide rails are oppositely arranged on the side wall of the cup placing slot to be matched with cup clamping elastomers; the matching positions of the cup clamping elastomers and a cup are set into arc surfaces, and a plurality of clamping slots or raised lines are respectively formed in or arranged on the side faces of the cup clamping elastomers matched with the limiting guide rails; the cup clamping elastomers are matched with the limiting guide rails through the different clamping slots or raised lines, so that the cup placing slot is divided into two cup containing spaces with the variable inner diameters; and the outer end of the armrest cup holder structure is set as an incline plane. The automotive rear seat armrest cup holder structure has the advantages of simple structure, convenience in mounting, high stability and reliability and overall aesthetic coordination.