The invention provides an air-conditioner, a control method thereof and a multi-split air-conditioning system. An inlet of a nanometer water ion module in the air-conditioner communicates with a high-static-pressure area of the air-conditioner. A conveying pipe conveys electrified water ions to a low-static-pressure area, so that the electrified water ions are conveyed to a wind outlet under the effect of the static pressure difference and further conveyed to a room through the wind outlet, the electrified water ions cannot overcome the static pressure difference voluntarily to flow back intoa shell, the situation that the electrified water ions accumulate on the periphery of the shell and generate electromagnetic interference to the air-conditioner is avoided, and the air-conditioner hashigh operation stability and safety. According to the control method of the air-conditioner, the air-conditioner is controlled by means of the manner of controlling a fan prior to starting of the nanometer water ion module, and the stability and the safety of operation of the air-conditioner are improved. The multi-split air-conditioning system has high operation stability and safety.