The utility model relates to a rail support device of a sliding door. The rail support device can be applied to various sliding doors with rails, particularly can be applicable to the sliding doors for a marine petroleum exploitation platform, solves the problems that the sliding doors are corroded by substances such as seawater, atmosphere and alkali salt during use so that the sliding doors are in failure and can't be opened and are high in maintenance rate, and accordingly guarantees safety use of the sliding door and brings convenience to production and management. The rail support device of the sliding door comprises a rolling element and a bottom plate sleeve, a rolling element fixing post is welded on a rear back plate of the bottom plate sleeve, the rolling element is sleeved on the rolling element fixing post, the rolling element is enclosed in the bottom plate sleeve by a front cap plate, so that the rolling element is comprehensively protected. The rail support device of the sliding door is simple in structure, durable in use, reliable in operation and flexible in sliding, and the rolling element is comprehensively protected. The rail support device of the sliding door is applicable to various sliding doors with rails and is fine in use effect.