A slide rail assembly, comprising: a first rail having a front end and a rear end; a second rail displaceable with respect to the first rail; a third rail displaceable with respect to the second rail, the third rail having a front end and a rear end; and a first engaging mechanism movably mounted on the second rail and including a first engaging member and a first elastic member for applying an elastic force to the first engaging member; wherein the rear end of the first rail and the front end of the third rail define a first length therebetween when the second rail is displaced from a first position to a second position with respect to the first rail in a first direction and the third rail is displaced to a third position with respect to the second rail in the first direction and therefore extends out of the second rail; wherein when the second rail is displaced from the second position to a fourth position in a second direction, the first engaging member is tilted by an angle under the guidance of a guiding portion and is pressed against an engaging portion of a first stopping member of the first rail in response to an elastic force of the first elastic member, the first length between the rear end of the first rail and the front end of the third rail is reduced to a second length, and the second rail is kept at the fourth position with respect to the first rail by the first engaging mechanism, wherein the slide rail assembly further comprises a first working member, a second working member and a switching member mounted on the third rail, and a stopping base arranged on the second rail has a stopping portion, wherein the switching member is displaceable between a first switching position and a second switching position by user operation, in which the switching member is not aligned with the stopping portion of the stopping base when in the first switching position but is aligned with the stopping portion of the stopping base when in the second switching position, and wherein when the third rail is displaced in the second direction and the switching member is in the second switching position, the switching member is adapted to push a side of the stopping portion of the stopping base such that the second rail is displaceable along with the third rail in the second direction from the second position. a first rail having a front end and a rear end; a second rail displaceable with respect to the first rail; a third rail displaceable with respect to the second rail, the third rail having a front end and a rear end; and a first engaging mechanism movably mounted on the second rail and including a first engaging member and a first elastic member for applying an elastic force to the first engaging member; wherein the rear end of the first rail and the front end of the third rail define a first length therebetween when the second rail is displaced from a first position to a second position with respect to the first rail in a first direction and the third rail is displaced to a third position with respect to the second rail in the first direction and therefore extends out of the second rail; wherein when the second rail is displaced from the second position to a fourth position in a second direction, the first engaging member is tilted by an angle under the guidance of a guiding portion and is pressed against an engaging portion of a first stopping member of the first rail in response to an elastic force of the first elastic member, the first length between the rear end of the first rail and the front end of the third rail is reduced to a second length, and the second rail is kept at the fourth position with respect to the first rail by the first engaging mechanism, wherein the slide rail assembly further comprises a first working member, a second working member and a switching member mounted on the third rail, and a stopping base arranged on the second rail has a stopping portion, wherein the switching member is displaceable between a first switching position and a second switching position by user operation, in which the switching member is not aligned with the stopping portion of the stopping base when in the first switching position but is aligned with the stopping portion of the stopping base when in the second switching position, and wherein when the third rail is displaced in the second direction and the switching member is in the second switching position, the switching member is adapted to push a side of the stopping portion of the stopping base such that the second rail is displaceable along with the third rail in the second direction from the second position.