Slide rail assembly comprising a rail and a bracket device applicable to the rail, the bracket device comprising: a first supporting bracket connected to the rail of the slide rail assembly; a second supporting bracket movably connected to the first supporting bracket; a third supporting bracket movably connected to the second supporting bracket; and a mounting bracket movably connected to the third supporting bracket and comprising a side plate, a first bent portion and a second bent portion respectively connected to two opposite sides of the side plate, and a bracket passage, which is defined between the first bent portion and the second bent portion, wherein the side plate of the mounting bracket further comprises a first blocking portion and a second blocking portion, and a slot, which is defined between the first blocking portion of the mounting bracket and the second blocking portion of the mounting bracket; and the bracket device further comprises a connecting member passing through the slot of the side plate of the mounting bracket and being connected to the third supporting bracket, allowing the mounting bracket to be adjusted relative to the third supporting bracket to a limited extent by the length of the slot characterized in, that the rail is inserted in the bracket passage such that the mounting bracket is mounted to the rail and displaceable relative to the rail, and that the rail further includes a first blocking portion and a second blocking portion, and a slot, which is defined between the first blocking portion of the rail and the second blocking portion of the rail, and that the bracket device further includes a protruding member passing through the slot of the rail and being connected to the second supporting bracket, allowing the second supporting bracket to be adjusted relative to the first supporting bracket to a limited extend defined by the length of the slot, and that the second supporting bracket comprises a blocking section, and that the third supporting bracket comprises a blocking section corresponding to the blocking section of the second supporting bracket, so that during an adjustment, the blocking section of the third supporting bracket is pressed against the blocking section of the second supporting bracket and drives the second supporting bracket into displacement. a first supporting bracket connected to the rail of the slide rail assembly; a second supporting bracket movably connected to the first supporting bracket; a third supporting bracket movably connected to the second supporting bracket; and a mounting bracket movably connected to the third supporting bracket and comprising a side plate, a first bent portion and a second bent portion respectively connected to two opposite sides of the side plate, and a bracket passage, which is defined between the first bent portion and the second bent portion, wherein the side plate of the mounting bracket further comprises a first blocking portion and a second blocking portion, and a slot, which is defined between the first blocking portion of the mounting bracket and the second blocking portion of the mounting bracket; and the bracket device further comprises a connecting member passing through the slot of the side plate of the mounting bracket and being connected to the third supporting bracket, allowing the mounting bracket to be adjusted relative to the third supporting bracket to a limited extent by the length of the slot characterized in, that the rail is inserted in the bracket passage such that the mounting bracket is mounted to the rail and displaceable relative to the rail, and that the rail further includes a first blocking portion and a second blocking portion, and a slot, which is defined between the first blocking portion of the rail and the second blocking portion of the rail, and that the bracket device further includes a protruding member passing through the slot of the rail and being connected to the second supporting bracket, allowing the second supporting bracket to be adjusted relative to the first supporting bracket to a limited extend defined by the length of the slot, and that the second supporting bracket comprises a blocking section, and that the third supporting bracket comprises a blocking section corresponding to the blocking section of the second supporting bracket, so that during an adjustment, the blocking section of the third supporting bracket is pressed against the blocking section of the second supporting bracket and drives the second supporting bracket into displacement.