The invention discloses a system and a method for treating low-rank coal. The system comprises a hot blast composite gangue removing device with a low-rank coal inlet, a separation hot blast inlet, a gangue outlet, and a cleaned coal outlet, a screening device with a cleaned coal inlet, an oversize cleaned coal outlet, and an undersize cleaned coal outlet, a pyrolysis device with a feed inlet connected with the oversize cleaned coal outlet, a semicoke outlet, and a pyrolytic oil gas outlet, a drying device with a dry hot blast inlet, an undersize cleaned coal inlet connected with the undersize cleaned coal outlet, and a dry pulverized coal outlet, a waste heat recovery device with a semicoke inlet, a cooled semicoke outlet, and a hot gas outlet connected with the separation hot blast inlet and the dry hot blast outlet, and a pulverized coal combustion power generation device with a pulverized coal inlet connected with the dry pulverized coal outlet. The system provides a new way for high-efficiency conversion and utilization of the low-rank coal by integrating the gangue removal, drying, pyrolysis and combustion power generation technologies of the low-rank coal.