A method for positioning an endoscope involves installing an auxiliary positioning device on a robot arm, coinciding a terminal of a docking member of the auxiliary positioning device with a spherical remote center of motion defined by the robot arm, inserting and fixing the endoscope inside the auxiliary positioning device, removing the docking member of the auxiliary positioning device so that a terminal of the endoscope coincides with the spherical remote center of motion of the robot arm, and then inserting the endoscope into a body cavity catheter for finalizing positioning. Thereby, the method helps to save time used for preoperative preparation and provides more precise positioning, without using any additional positioning tools to approach the body cavity, thereby reducing the risk of infection.