A rail mounting adapter assembly configured to mount a slide rail assembly (26, 28) on a rack, the rack having a plurality of first mounting features and a plurality of second mounting features arranged on a side of the rack, the rail mounting adapter assembly comprising: a first base comprising a first side plate and a first end plate perpendicularly connected to the first side plate, wherein the first side plate is detachably mounted at a first position on the side of the rack, the first end plate has a plurality of first mounting holes, the plurality of first mounting holes being configured to be mounted with an end of the slide rail assembly(26, 28); a second base comprising a second side plate and a second end plate, wherein the second side plate is detachably mounted at a second position on the side of the rack, the second end plate corresponds to the first end plate of the first base, the second end plate has a plurality of second mounting holes, the plurality of second mounting holes being configured to be mounted with an end of the slide rail assembly(26, 28); and a supporting body extendedly connected between the first base and the second base; wherein the rail mounting adapter assembly further comprising a fastening member mounted on the first base, wherein the fastening member comprises an elastic part, an engaging parts (52a, 52b), an operating part tilted relative to the elastic part, and an extension part connected to the elastic part and protruded relative to the elastic part, the elastic part is configured to provide an elastic force, the engaging parts (52a, 52b) are respectively engaged with one and the other one of the first mounting features on the side of the rack in response to the elastic force of the elastic part, and the operating part is configured to be operated such that the elastic part allows the first engaging part, the second engaging part and the extension part to be disengaged from the first mounting features, wherein the first mounting features comprises a first hole. a first base comprising a first side plate and a first end plate perpendicularly connected to the first side plate, wherein the first side plate is detachably mounted at a first position on the side of the rack, the first end plate has a plurality of first mounting holes, the plurality of first mounting holes being configured to be mounted with an end of the slide rail assembly(26, 28); a second base comprising a second side plate and a second end plate, wherein the second side plate is detachably mounted at a second position on the side of the rack, the second end plate corresponds to the first end plate of the first base, the second end plate has a plurality of second mounting holes, the plurality of second mounting holes being configured to be mounted with an end of the slide rail assembly(26, 28); and a supporting body extendedly connected between the first base and the second base; wherein the rail mounting adapter assembly further comprising a fastening member mounted on the first base, wherein the fastening member comprises an elastic part, an engaging parts (52a, 52b), an operating part tilted relative to the elastic part, and an extension part connected to the elastic part and protruded relative to the elastic part, the elastic part is configured to provide an elastic force, the engaging parts (52a, 52b) are respectively engaged with one and the other one of the first mounting features on the side of the rack in response to the elastic force of the elastic part, and the operating part is configured to be operated such that the elastic part allows the first engaging part, the second engaging part and the extension part to be disengaged from the first mounting features, wherein the first mounting features comprises a first hole.