The invention discloses a duct type air conditioning system and a control method and device for an indoor fan thereof. The method comprises the following steps that when the duct type air conditioning system is started, an indoor environment temperature and a set temperature recorded when the duct type air conditioning system is stopped last time are obtained, and a set temperature this time is obtained according to the indoor environment temperature and the set temperature recorded when the system is stopped last time; the indoor environment temperature is detected in real time; a temperature difference between the current indoor environment temperature and the set temperature is calculated according to the current indoor environment temperature and the set temperature this time, and a wind speed regulating instruction is generated according to the temperature difference; and the indoor fan is controlled according to the wind speed regulating instruction, so as to regulate the air capacity of the indoor fan. Accordingly, the set temperature is obtained through a certain operating rule when the indoor fan fails to receive parameters of the set temperature and the environment temperature, and the air capacity is automatically controlled; and meanwhile, dehumidification of an air conditioner is improved, and the comfort and the energy-saving performance of the air conditioner are improved.