The invention discloses a multi-online system and a control method for a heating and throttling element of the multi-online system. The method comprises the following steps that the target exhaust pressure of a compressor or the saturation temperature corresponding to the target exhaust pressure is obtained; the compressor is controlled according to the target exhaust pressure or the saturation temperature corresponding to the target exhaust pressure, and the current exhaust overheat degree of the compressor is obtained after the compressor stably runs; the target exhaust overheat degree of the compressor is obtained, and whether the current exhaust overheat degree is smaller than the target exhaust overheat degree or not is judged; and if the current exhaust overheat degree is smaller than the target exhaust overheat degree, aperture decreasing control over the heating and throttling element is conducted till the difference value between the high-pressure pressure and the intermediate-pressure pressure of a diversion device is smaller than the first preset value or when a liquid accumulation signal of a heating indoor unit is received, the aperture decreasing of the heating and throttling element is forbidden, and therefore it is guaranteed that the system has the high overheat degree. When the system conducts heating, especially in heating of part of loads, better performance and higher energy efficiency are achieved.