A slide rail assembly, comprising: a first rail; a second rail; a mounting member comprising a first part, a second part and a third part, wherein the first part and the second part are connected to the third part and respectively extended along different directions; a third rail connected to the first part of the mounting member, wherein the third rail is movable relative to the first rail; a fourth rail connected to the second part of the mounting member, wherein the second rail is movable relative to the fourth rail; an engaging member movably mounted to the mounting member; and an elastic member configured to provide an elastic force to the engaging member; wherein when the second rail is moved from a retracted position to a first predetermined position along a first direction, the engaging member is engaged with the second rail in response to the elastic force of the elastic member, so as to allow the mounting member to be synchronously moved with the second rail along the first direction; wherein the fourth rail comprises a first wall, a second wall and a side wall connected between the first wall and the second wall of the fourth rail, the second rail comprises a first wall, a second wall and a side wall connected between the first wall and the second wall of the second rail, the engaging member is pivoted to the mounting member, wherein one of the first wall and the second wall of the second rail has an abutting part; when the second rail is located at the first predetermined position, the engaging part of the engaging member contacts the abutting part, in order to allow the second rail to drive the mounting member to move along the first direction through the abutting part, characterized in that one of the first wall and the second wall of the fourth rail has a notch, the notch is configured to allow an engaging part of the engaging member to insert therein. a first rail; a second rail; a mounting member comprising a first part, a second part and a third part, wherein the first part and the second part are connected to the third part and respectively extended along different directions; a third rail connected to the first part of the mounting member, wherein the third rail is movable relative to the first rail; a fourth rail connected to the second part of the mounting member, wherein the second rail is movable relative to the fourth rail; an engaging member movably mounted to the mounting member; and an elastic member configured to provide an elastic force to the engaging member; wherein when the second rail is moved from a retracted position to a first predetermined position along a first direction, the engaging member is engaged with the second rail in response to the elastic force of the elastic member, so as to allow the mounting member to be synchronously moved with the second rail along the first direction; wherein the fourth rail comprises a first wall, a second wall and a side wall connected between the first wall and the second wall of the fourth rail, the second rail comprises a first wall, a second wall and a side wall connected between the first wall and the second wall of the second rail, the engaging member is pivoted to the mounting member, wherein one of the first wall and the second wall of the second rail has an abutting part; when the second rail is located at the first predetermined position, the engaging part of the engaging member contacts the abutting part, in order to allow the second rail to drive the mounting member to move along the first direction through the abutting part, characterized in that one of the first wall and the second wall of the fourth rail has a notch, the notch is configured to allow an engaging part of the engaging member to insert therein.