A method for controlling a host apparatus, includes steps of the followings. First, a pattern is projected to a detection region. Second, images of the detection region are captured and image information stream is generated based on the images. Third, an image information associated with the pattern is identified to be a background image information. Fourth, a currently captured image information is subtracted to the background image information to identify if an object appears in the detection region. Fifth, a motion of the identified object is determined based on a variation of the pattern reflected from the identified object to generate a control instruction to the host apparatus. A host apparatus, an interaction control system and a method for detecting a motion of an object are also provided.