A rack mounting system is applicable to a rack. The rack mounting system includes a mounting mechanism, a first connecting assembly and a second connecting assembly. The mounting mechanism is detachably mounted to a first side of the rack. The first connecting assembly is detachably mounted to a second side of the rack. The second connecting assembly is connected to the mounting mechanism. Wherein, the mounting mechanism includes a main body and at least one pin, the main body is configured to be detachably mounted to the rack. The at least one pin is slidably passing through the main body, so that the at least one pin is movable relative to the rack. Wherein, the first connecting assembly and the second connecting assembly are configured to carry a first chassis with a first chassis width when the at least one pin is moved to a first position.