A cylindrical lithium battery cap sealing structure with a built-in BMS board includes a steel housing and a cap mechanism. An outer surface of a side of the steel housing is fixedly connected to a bottom housing, and a top surface of the bottom housing is fixedly provided with a battery roll core adapted to the steel housing. An outer surface of a side of the cap mechanism is fixedly connected to a connecting housing, and an outer surface of a side of the connecting housing is integrally connected to an outer surface of a side of the steel housing. The mainboard, negative hardware ring, and positive aluminum plate in the cylindrical lithium battery cap sealing structure with a built-in BMS board are all circular. A plastic bracket is annular and has a sealing adhesive groove, when matched with the steel housing, a complete set of annular structures is formed.