A manufacturing method of a turbomachine airfoil, such as an outlet guide vane airfoil, comprising positioning a first fibrous wall preform on a first mold portion, placing at least one core on the first wall preform, positioning a second fibrous wall preform on the core, assembling a second mold portion to the first mold portion so as to form a mold around the first and second wall preforms, applying a hardening treatment to the first and second wall preforms, removing the core, and positioning a reinforcing structure between the first wall preform and the second wall preform.