An online oil coating device of a cold rolled extremely thin steel belt comprises a felt, a nozzle, an oil pipe and an oil storage hopper. The oil pipe is arranged at the lower portion of the oil storage hopper and connected to the nozzle arranged in the front of a tension platform of a slitting unit. The felt is installed below the tension platform of the slitting unit and tightly attached to the upper surface and the lower surface of the steel belt. As a further improvement of the online oil coating device, the oil pipe arranged at the lower portion of the oil storage hopper is provided with a stop valve. The online oil coating device well overcomes the shortcoming of the existing manual coating way; solves the problem that due to the fact that the thickness of an oil layer is not easy to control, antirust oil is not coated on some parts, and the antirust oil of some parts gathers too much; meets the requirement for surface protection of the cold rolled extremely thin steel belt; avoids environmental pollution; reduces oil consumption; and improves efficiency.