The invention discloses a multi-unit system and a control method of a heating indoor unit thereof. The method comprises the following steps: a shunt device obtains the target opening and the target duration of a throttling element in an indoor unit according to a preset opening and duration table if the indoor unit which is not started or at a refrigeration mode receives heating mode commands when the multi-unit system is at a pure refrigeration mode or a main refrigeration mode, controls the throttling element in the indoor unit to be opened to the target opening to adjust the pressure in front and rear of the throttling element, and adjusts the opening of the throttling element to the maximum opening when the target opening is operated for the target duration; a refrigeration control valve corresponding to the indoor unit is controlled to be closed after time is delayed by a first preset time; a corresponding heating control valve is controlled to be opened after time is delayed by a second preset time. The method avoids scouring noise produced when refrigerant flows rapidly and the influence on the refrigeration capacity of other indoor units by performing pressure equalizing on the indoor unit receiving the heating mode commands.