A slide rail assembly adapted for a rack, the slide rail assembly comprising: a first rail movably mounted on the rack; a second rail movably mounted on the first rail, wherein the second rail is longitudinally displaceable with respect to the first rail, and the second rail includes a first end portion, a second end portion, and a channel located between the first end portion and the second end portion; a fitting including at least one guiding structure (54a, 54b, 56) corresponding to the channel of the second rail; and a third rail; wherein the third rail is able to be guided by the at least one guiding structure (54a, 54b, 56) of the fitting while being outside the channel of the second rail and thus enter the channel of the second rail; wherein the second rail includes a first wall, a second wall, and a longitudinal wall connected between the first wall and the second wall of the second rail; the first wall, the second wall, and the longitudinal wall of the second rail jointly define the channel; the fitting includes a first portion, a second portion, and a side portion connected between the first portion and the second portion; and the first portion, the second portion, and the side portion jointly define an auxiliary channel corresponding to the channel of the second rail; characterized in that the first rail is transversely movable with respect to the rack between a first position (P 1) and a second position; wherein the fitting has two opposite sides defined respectively as a first side (S 1) and a second side, the auxiliary channel and the at least one guiding structure (54a, 54b, 56) are located on the first side (S 1), the fitting is further provided with a predetermined structure on the second side, and the predetermined structure is configured for contact with the rack; wherein the predetermined structure is an elastic section; wherein the predetermined structure enables the fitting to adapt to the first rail being at the second position with respect to the rack, or at the first position (P 1) with respect to the rack, or at any position between the first position and the second position, so that the fitting can be mounted on the second rail. a first rail movably mounted on the rack; a second rail movably mounted on the first rail, wherein the second rail is longitudinally displaceable with respect to the first rail, and the second rail includes a first end portion, a second end portion, and a channel located between the first end portion and the second end portion; a fitting including at least one guiding structure (54a, 54b, 56) corresponding to the channel of the second rail; and a third rail; wherein the third rail is able to be guided by the at least one guiding structure (54a, 54b, 56) of the fitting while being outside the channel of the second rail and thus enter the channel of the second rail; wherein the second rail includes a first wall, a second wall, and a longitudinal wall connected between the first wall and the second wall of the second rail; the first wall, the second wall, and the longitudinal wall of the second rail jointly define the channel; the fitting includes a first portion, a second portion, and a side portion connected between the first portion and the second portion; and the first portion, the second portion, and the side portion jointly define an auxiliary channel corresponding to the channel of the second rail; characterized in that the first rail is transversely movable with respect to the rack between a first position (P 1) and a second position; wherein the fitting has two opposite sides defined respectively as a first side (S 1) and a second side, the auxiliary channel and the at least one guiding structure (54a, 54b, 56) are located on the first side (S 1), the fitting is further provided with a predetermined structure on the second side, and the predetermined structure is configured for contact with the rack; wherein the predetermined structure is an elastic section; wherein the predetermined structure enables the fitting to adapt to the first rail being at the second position with respect to the rack, or at the first position (P 1) with respect to the rack, or at any position between the first position and the second position, so that the fitting can be mounted on the second rail.