Carrier multiplication (CM), which generates multiexcitons from a single photon absorption, is advantageous for increasing the optoelectronic device efficiency. However, CM is intrinsically inefficient in conventional semiconductors, and enhancing CM has been a long-standing challenge. Here, we propose that multisubbands in nanostructures can significantly enhance CM by opening up the intersubband CM transitions, which circumvent the strict restrictions enforced by the energy and momentum conservations. Using real-time time-dependent density functional theory, we demonstrate the mechanism in graphene nanoribbons as an example of a multisubband system. The CM mechanism provides a pathway for developing efficient optoelectronic devices.