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. <EOS>