A fast positioning structure of a safe position limiting device of a roller blind, comprising: a controller having an axle rod, wherein the axle rod is further provided with a through hole; an axial rail limiting assembly having a screw member, an elastic member, and an inner tube, wherein the screw member is provided with an axle hole on a first end surface thereof for engaging with the axle rod for positioning, and a first chamber in a second end surface thereof for allowing corresponding insertion of the inner tube and horizontal movement of the inner tube therein, and wherein the inner tube is provided with a central through hole on an end surface thereof and a second chamber therein for communicating with the central through hole and accommodating the elastic member; a touch member for correspondingly inserting into the central through hole of the inner tube, the elastic member, the screw member and the through hole in the controller for positioning and for compressing the elastic member to limit the inner tube within the first chamber of the screw member or release the inner tube for horizontal movement; a positioning member; characterized in that said fast positioning structure further comprises a position limiting member screwed at the external of the screw member and having a plurality of engaging blocks at an outer edge thereof for correspondingly engaging with a curtain tube disposed with a curtain fabric, wherein the position limiting member further defines a second locking block at one end thereof; and an outer sleeve tube having an inner tooth ring for correspondingly screwing at the external of the screw member, the outer sleeve tube further defining a jointing block at one end thereof configured for correspondingly engaging with the second locking block of the position limiting member,the outer sleeve tube further including a seal cap at the other end thereof, wherein an end surface of the seal cap is provided with a first locking structure correspronding to a second locking structure on the end surface of the inner tube for correspondingly locking the inner tube in the outer sleeve tube or separating the inner tube from the outer sleeve tube, whereby the positioning member is screwed at an external of the screw member, is disposed at one side of the positioning limiting member and has an outer ring gear disposed at the external of the screw member and further having a lug for moving on a trench at the first end of the screw member, and a positioning ring further having a tooth ring for screwing with the screw member, an inner ring gear for screwing with the outer ring gear; and an engaging tooth part corresponding to an outer edge of the tooth ring for engaging with a first locking block of the position limiting member. a controller having an axle rod, wherein the axle rod is further provided with a through hole; an axial rail limiting assembly having a screw member, an elastic member, and an inner tube, wherein the screw member is provided with an axle hole on a first end surface thereof for engaging with the axle rod for positioning, and a first chamber in a second end surface thereof for allowing corresponding insertion of the inner tube and horizontal movement of the inner tube therein, and wherein the inner tube is provided with a central through hole on an end surface thereof and a second chamber therein for communicating with the central through hole and accommodating the elastic member; a touch member for correspondingly inserting into the central through hole of the inner tube, the elastic member, the screw member and the through hole in the controller for positioning and for compressing the elastic member to limit the inner tube within the first chamber of the screw member or release the inner tube for horizontal movement; a positioning member; characterized in that said fast positioning structure further comprises a position limiting member screwed at the external of the screw member and having a plurality of engaging blocks at an outer edge thereof for correspondingly engaging with a curtain tube disposed with a curtain fabric, wherein the position limiting member further defines a second locking block at one end thereof; and an outer sleeve tube having an inner tooth ring for correspondingly screwing at the external of the screw member, the outer sleeve tube further defining a jointing block at one end thereof configured for correspondingly engaging with the second locking block of the position limiting member,the outer sleeve tube further including a seal cap at the other end thereof, wherein an end surface of the seal cap is provided with a first locking structure correspronding to a second locking structure on the end surface of the inner tube for correspondingly locking the inner tube in the outer sleeve tube or separating the inner tube from the outer sleeve tube, whereby the positioning member is screwed at an external of the screw member, is disposed at one side of the positioning limiting member and has an outer ring gear disposed at the external of the screw member and further having a lug for moving on a trench at the first end of the screw member, and a positioning ring further having a tooth ring for screwing with the screw member, an inner ring gear for screwing with the outer ring gear; and an engaging tooth part corresponding to an outer edge of the tooth ring for engaging with a first locking block of the position limiting member.