The embodiment of the invention provides a multiple on-line self-cleaning method and a multi-split air conditioning system. The multiple on-line self-cleaning method is applied to the multi-split airconditioning system, and the multi-split air conditioning system comprises a plurality of indoor units. The multiple on-line self-cleaning method includes the steps that the indoor units are controlled to start a cooling mode according to a preset compressor frequency, and fans of the indoor units are controlled to stop running; average evaporation temperature information corresponding to the indoor units in the cooling mode is acquired according to a preset time period; the compressor frequency is adjusted according to the average evaporation temperature information, so that the indoor unitsall satisfy a preset refrigeration exit condition; and the indoor units are controlled to start a heating mode to complete cleaning. According to the scheme, the compressor frequency is adjusted according to the average evaporation temperature information of the indoor units which still operate in the cooling mode, it is ensured that each indoor unit can reach the ideal frosting effect, and the final cleaning effect of each indoor unit is ensured.