The invention provides a system and a method for controlling the constant air volume of an air duct type air conditioning indoor unit. The method comprises the following steps of: outputting a constant duty ratio D by a central processing unit to drive a direct current motor; detecting the rotating speed of the direct current motor by a rotating speed detection module, and transmitting the rotating speed to a rotating speed comparison module; comparing the rotating speed with a motor rotating speed value of a rotating speed unit by the rotating speed comparison module to obtain static pressure, and outputting the static pressure to a storage module; and contrasting a required constant air volume and static pressure with the data stored in the target rotating speed unit by the central processing unit to obtain a motor rotating speed value, and controlling the direct current motor to reach the rotating speed value. According to the system and the method for controlling the constant air volume of the air duct type air conditioning indoor unit, the automatic identification of the static pressure can be realized according to different air duct lengths so as to control the constant air volume, so that the output of the constant air volume can be realized under any static pressure conditions.