A chair backrest folding locking mechanism is provided, which includes a fixing bracket, defining a first locking hole and a second locking hole disposed transversely; a chair backrest, hinged with the fixing bracket; a transverse locking member, disposed on the chair backrest, where an outer end of the transverse locking member extends laterally from a lower end of the chair backrest, and the transverse locking member telescopically moves relative to the chair backrest in a transverse direction; a driving member, driving the outer end of the transverse locking member to extend into either the first locking hole or the second locking hole; a front side resisting part, disposed on the fixing bracket and below the chair backrest, and located at a side of the chair backrest proximate to a human back; and a rear side resisting part, disposed on the lower end of the chair backrest.