A defrosting control method for multi-split system, wherein the multi-split system includes an outdoor unit and a plurality of indoor units (1, 2, 3... N), an expansion valve is disposed on each of connecting pipes between one of the indoor units and the outdoor unit, the defrosting control method including: determining whether or not satisfying a defrost condition; if satisfying the defrost condition, starting a defrost mode; characterised in the defrosting control method further including: if satisfying the defrost condition, further determining whether or not satisfying a defrosting requirement under an assumption that the expansion valves on working indoor units are closed and the opening degree of each of the expansion valves of off-state indoor units ≤ a maximum set opening degree offLimitMaxPLS; if satisfying the defrosting requirement, closing the expansion valves of the working indoor units and adjusting the opening degree of each of the expansion valves of the off-state indoor units to Off_PLS, wherein Off_PLS = ALL_HP*Avg_PLS / Off_HP; in which; Off_HP denoting a total capacity of the off-state indoor units; Off_PLS denoting an opening degree of each of the expansion valves of the off-state indoor units; offLimitMaxPLS denoting a maximum set opening degree of each of the expansion valves on the off-state indoor units; ALL_HP denoting a total capacity of the plurality of indoor units, and Avg_PLS denoting a set average opening degree, which indicates that if the opening degree of each of the expansion valves of the plurality of indoor units is the set average opening degree the multi-split system satisfying the defrosting requirement. determining whether or not satisfying a defrost condition; if satisfying the defrost condition, starting a defrost mode; characterised in the defrosting control method further including: if satisfying the defrost condition, further determining whether or not satisfying a defrosting requirement under an assumption that the expansion valves on working indoor units are closed and the opening degree of each of the expansion valves of off-state indoor units ≤ a maximum set opening degree offLimitMaxPLS; if satisfying the defrosting requirement, closing the expansion valves of the working indoor units and adjusting the opening degree of each of the expansion valves of the off-state indoor units to Off_PLS, wherein Off_PLS = ALL_HP*Avg_PLS / Off_HP; in which; Off_HP denoting a total capacity of the off-state indoor units; Off_PLS denoting an opening degree of each of the expansion valves of the off-state indoor units; offLimitMaxPLS denoting a maximum set opening degree of each of the expansion valves on the off-state indoor units; ALL_HP denoting a total capacity of the plurality of indoor units, and Avg_PLS denoting a set average opening degree, which indicates that if the opening degree of each of the expansion valves of the plurality of indoor units is the set average opening degree the multi-split system satisfying the defrosting requirement. if satisfying the defrost condition, further determining whether or not satisfying a defrosting requirement under an assumption that the expansion valves on working indoor units are closed and the opening degree of each of the expansion valves of off-state indoor units ≤ a maximum set opening degree offLimitMaxPLS; if satisfying the defrosting requirement, closing the expansion valves of the working indoor units and adjusting the opening degree of each of the expansion valves of the off-state indoor units to Off_PLS, wherein Off_PLS = ALL_HP*Avg_PLS / Off_HP; in which; Off_HP denoting a total capacity of the off-state indoor units; Off_PLS denoting an opening degree of each of the expansion valves of the off-state indoor units; offLimitMaxPLS denoting a maximum set opening degree of each of the expansion valves on the off-state indoor units; ALL_HP denoting a total capacity of the plurality of indoor units, and Avg_PLS denoting a set average opening degree, which indicates that if the opening degree of each of the expansion valves of the plurality of indoor units is the set average opening degree the multi-split system satisfying the defrosting requirement. Off_HP denoting a total capacity of the off-state indoor units; Off_PLS denoting an opening degree of each of the expansion valves of the off-state indoor units; offLimitMaxPLS denoting a maximum set opening degree of each of the expansion valves on the off-state indoor units; ALL_HP denoting a total capacity of the plurality of indoor units, and Avg_PLS denoting a set average opening degree, which indicates that if the opening degree of each of the expansion valves of the plurality of indoor units is the set average opening degree the multi-split system satisfying the defrosting requirement.