The invention relates to a system and a method for deeply purifying coal. The system is composed of a flotation column, a mixing tank, a circulating pump, a bubble generator, an oil storage barrel, a gasoline pump and an oil gas generator. The method comprises the following steps: adding kerosene or diesel oil into the oil gas generator and then heating and evaporating the kerosene or diesel oil,thereby generating kerosene or diesel oil steam; introducing the kerosene or diesel oil steam together with air into a floatation device, thereby generating oil bubbles; and promoting the wetting property of the bubbles under the condition of maintaining the floatation of a mineral main body, thereby achieving the purpose of promoting the floatability of the mineral. The problem of difficulty in flotation of low-rank coals can be solved, the rate of multipurpose utilization of the coal can be increased and the economic value can be promoted. The system and method provided by the invention aresuitable for deeply purifying the low-rank coals with low metamorphic grade, such as brown coal, long flame coal, non-caking coal, weakly caking coal, gas coal, and the like, can be used for greatly promoting the yield of clean coal and can be used for obviously reducing the dosage of the kerosene or diesel oil used as a collecting agent. The system has the advantages that the structure is simple, the operation is convenient, the floatation index of the low-rank coals is excellent and the dosage is small.