A duct-type air conditioning system, a control method and a device for an indoor ventilator thereof. The control method comprises the following steps: upon turning on the duct-type air conditioning system, acquiring an indoor temperature and a set temperature recorded the last time the duct-type air conditioning system was turned off, and acquiring a present set temperature (S1) according to the indoor temperature and the set temperature recorded the last time the system was turned off; detecting a real-time indoor temperature, according to the present indoor temperature and the present set temperature, calculating a temperature difference between the present indoor temperature and the set temperature, and generating an air velocity adjustment instruction according to the temperature difference, controlling, according to the air velocity adjustment instruction, the indoor ventilator to adjust the air supply velocity (S4) of an indoor unit. Thus, the invention acquires a set temperature by means of certain operating rules to achieve automatic control of air supply velocity when the indoor unit cannot receive the parameters of a set temperature and an ambient temperature.