1. A coordinate transformation method for a user and an interactive system comprising a detection module, the coordinate transformation method comprising: determining a face information and a command object of the user via the detection module to obtain a face coordinate and a command object coordinate; transforming the face coordinate into a transformed face coordinate according to a coordinate of the detection module; obtaining an angle between an optical-axis ray and a line formed via connecting the transformed face coordinate and the coordinate of the detection module; obtaining a transformed command object coordinate according to the angle and the command object coordinate; and determining a depth change of the command object according to the transformed command object coordinate to set up an interactive operation between the interactive system and the user.