A slide rail assembly mechanism, comprising: a first rail comprising a disengaging feature; a second rail, wherein an abutting feature is arranged on the second rail; a mounting member; a third rail and a fourth rail respectively connected to a first side of the mounting member and a second side of the mounting member; and a synchronizing member arranged on the mounting member; wherein the third rail is longitudinally movable relative to the first rail, and the second rail is longitudinally movable relative to the fourth rail; wherein when the second rail is moved from a retracted position along an extending direction, the abutting feature of the second rail abuts against the synchronizing member, such that the mounting member and the second rail are synchronously moved, when the mounting member and the second rail are synchronously moved to a predetermined position, the synchronizing member is driven by the disengaging feature of the first rail to be disengaged from the abutting feature, such that the second rail and the mounting member are no longer synchronously moved; characterized by the first rail further comprising an engaging feature, the slide rail assembly further comprising a positioning member able to be located at a disengaging position or an engaging position relative to the mounting member, when the mounting member and the second rail are no longer synchronously moved, the mounting member is engaged with the engaging feature of the first rail through the positioning member being moved from the disengaging position to the engaging position; and by the positioning member being movably mounted on the mounting member, the mounting member having a second opening corresponding to the first rail, a part of the positioning member passes through the second opening to be engaged with the engaging feature of the first rail, wherein the slide rail assembly further comprises a second elastic member configured to provide an elastic force to the positioning member. a first rail comprising a disengaging feature; a second rail, wherein an abutting feature is arranged on the second rail; a mounting member; a third rail and a fourth rail respectively connected to a first side of the mounting member and a second side of the mounting member; and a synchronizing member arranged on the mounting member; wherein the third rail is longitudinally movable relative to the first rail, and the second rail is longitudinally movable relative to the fourth rail; wherein when the second rail is moved from a retracted position along an extending direction, the abutting feature of the second rail abuts against the synchronizing member, such that the mounting member and the second rail are synchronously moved, when the mounting member and the second rail are synchronously moved to a predetermined position, the synchronizing member is driven by the disengaging feature of the first rail to be disengaged from the abutting feature, such that the second rail and the mounting member are no longer synchronously moved; characterized by the first rail further comprising an engaging feature, the slide rail assembly further comprising a positioning member able to be located at a disengaging position or an engaging position relative to the mounting member, when the mounting member and the second rail are no longer synchronously moved, the mounting member is engaged with the engaging feature of the first rail through the positioning member being moved from the disengaging position to the engaging position; and by the positioning member being movably mounted on the mounting member, the mounting member having a second opening corresponding to the first rail, a part of the positioning member passes through the second opening to be engaged with the engaging feature of the first rail, wherein the slide rail assembly further comprises a second elastic member configured to provide an elastic force to the positioning member.