A slide rail assembly, comprising: a first rail comprising a releasing feature; a second rail movable relative to the first rail; a third rail movable relative to the second rail; and an engaging member pivoted to the second rail; wherein the engaging member engages with the third rail to allow the second rail and the third rail to be synchronously moved relative to the first rail along a first direction, and the releasing feature disengages the engaging member from the third rail when the second rail is synchronously moved with the third rail relative to the first rail to a predetermined position; wherein the third rail has a plurality of mounting holes (88a, 88b, 88c) spaced from each other, and the third rail comprises an extension feature for defining a vertical hole part and a longitudinal hole part of the mounting hole (88a, 88b, 88c), the longitudinal hole part is communicated with the vertical hole part; wherein a sliding auxiliary device is arranged between the second rail and the third rail, the sliding auxiliary device comprises a holding base and a plurality of rolling members arranged on the holding base, the plurality of rolling members are in rolling contact with the second rail and the third rail; wherein a first operating piece is mounted to the third rail, the first operating piece is operatively switched between a first working state and a second working state, the second rail comprises a blocking feature having a blocking section, when the engaging member is disengaged from the third rail by the releasing feature, and the third rail is moved relative to the second rail along the first direction to an extension position, the first operating piece abuts against one side of the blocking section of the blocking feature in the first working state; wherein the second rail has a first aperture and the sliding auxiliary device has a second aperture, when the third rail is located at the extension position, the first aperture, the second aperture and one of the mounting holes (88a, 88b, 88c) of the third rail are communicated with each other, and when the first operating piece is operatively switched from the first working state to the second working state, the third rail is detachable from the second rail along the first direction. a first rail comprising a releasing feature; a second rail movable relative to the first rail; a third rail movable relative to the second rail; and an engaging member pivoted to the second rail; wherein the engaging member engages with the third rail to allow the second rail and the third rail to be synchronously moved relative to the first rail along a first direction, and the releasing feature disengages the engaging member from the third rail when the second rail is synchronously moved with the third rail relative to the first rail to a predetermined position; wherein the third rail has a plurality of mounting holes (88a, 88b, 88c) spaced from each other, and the third rail comprises an extension feature for defining a vertical hole part and a longitudinal hole part of the mounting hole (88a, 88b, 88c), the longitudinal hole part is communicated with the vertical hole part; wherein a sliding auxiliary device is arranged between the second rail and the third rail, the sliding auxiliary device comprises a holding base and a plurality of rolling members arranged on the holding base, the plurality of rolling members are in rolling contact with the second rail and the third rail; wherein a first operating piece is mounted to the third rail, the first operating piece is operatively switched between a first working state and a second working state, the second rail comprises a blocking feature having a blocking section, when the engaging member is disengaged from the third rail by the releasing feature, and the third rail is moved relative to the second rail along the first direction to an extension position, the first operating piece abuts against one side of the blocking section of the blocking feature in the first working state; wherein the second rail has a first aperture and the sliding auxiliary device has a second aperture, when the third rail is located at the extension position, the first aperture, the second aperture and one of the mounting holes (88a, 88b, 88c) of the third rail are communicated with each other, and when the first operating piece is operatively switched from the first working state to the second working state, the third rail is detachable from the second rail along the first direction.