The utility model pertains to the technical field of vehicles and relates to an expanding device for top space of a compartment body of a vehicle. The expanding device comprises a chassis and the compartment body. Compartment body top plate cavities are formed on a top plate of the compartment body. The expanding device further comprises a cavity floating cover and a pair of floating cover on-off mechanisms arranged on the top plate of the compartment body. Floating cover on-off mechanisms are respectively connected with two sides corresponding to the cavity floating cover. The expanding device for the top space of the compartment body of the vehicle has following beneficial effects: height space of a compartment body cavity is substantially increased so that the compartment body cavity performs required motion conveniently; a sense of repression is eliminated due to the increase in space of the compartment body cavity; the cavity floating cover is closed on the compartment body top plate cavities in the non-enabled state so that a design standard is not deviated and safety is ensured; and due to the reasonable overall structure, an operating requirement for commercial activities is satisfied.