A production of thermoplastic composite preforms intended to be thermoformed. The unit including: a movable support and a plurality of storage areas for storing a plurality of structural elements made from thermoplastic composite material; and transfer and connection devices for transferring the structural elements to the movable support and connecting the structural elements to one another. The movable support is driven sequentially in translation in a direction of travel, while the storage areas are arranged laterally in relation to the movable support. The transfer and connection devices transfer the structural elements to the movable support along a transfer direction substantially perpendicular to the direction of travel.