An air cleaning system and its method of functioning are provided. The air cleaning system comprises an air cleaner and a charging base. The air cleaner comprises a control unit (1), an air cleaning unit (2), a travelling unit (3), an air pollution detection sensor (4) and a rechargeable battery (5). The working modes of the air cleaner comprise a charging mode and a travelling mode. The charging base is used to charge the rechargeable battery (5). The air pollution detection sensor (4) transmits the air pollution degree value to the control unit (1) in the charging mode. The control unit (1) comprises an electricity quantity detection subunit (11), an electricity quantity comparison subunit (12), an air pollution degree comparison subunit (13) and a treatment subunit (14). In the charging mode, based on the comparison results transmitted from the electricity quantity comparison subunit (12) and the air pollution degree comparison subunit (13), the treatment subunit (14) drives the air cleaning unit (2) to work when the electricity quantity value is greater than a first electricity quantity value and the air pollution degree value is greater than a first pollution value. The air cleaner is provided with the following advantages: allowing air cleaning in the charging mode, shortening the cleaning period, and increasing the cleaning efficiency.