The invention discloses a method for installing a motor of a belt fan in an air conditioner, which is characterized by comprising the following steps of (1) installing an auxiliary installation system according to the length of a transmission belt and the position of a fan; (2) evenly arranging an elastic member inside the auxiliary installation system; (3) fixing a motor onto a motor mounting plate; (4) aligning a connecting device at the edge of the motor mounting plate with a sliding channel on the inner wall of the auxiliary installation system and horizontally installing the connecting device onto the elastic member together with the motor; fixing the motor mounting plate and the elastic member; or fixing one side edge of the motor mounting plate to the auxiliary installation system via a rotating shaft, so as to enable the motor mounting plate to rotate freely; and then fixedly connecting the elastic member with the motor mounting plate; (5) connecting the fan with the motor via a transmission belt so as to complete the installation of the motor. Due to the adoption of the method for installing the motor in the air conditioner, the problems that the descending of transmission efficiency and shaking at the like caused by the high slack possibility of the transmission belt in the prior art are solved, the aim of ensuring the transmission belt to be always at a tense state is realized, besides, the buffer effect for the shaking of the electromotor can be generated effectively.