An apparatus for back-folding a standup seat of a vehicle may include a standup associated back-folding unit that is installed between a hook mounted to a lower frame of a seat cushion to be locked to or unlocked from a striker of a floor panel, and a recliner mounted to a side frame of a seatback such that a forward folding operation of the seatback and a forward folding operation of a headrest that is automatically performed by operating the recliner during a standup operation.