1. A method for teaching a robot arm to pick and place an object, comprising: pushing the robot arm until an object appears within a vision of an image capturing device; setting an appearance position of the object as a visual point; controlling the image capturing device to capture a first image at the visual point; pushing the robot arm to the object from the visual point; setting an arm position of the robot arm as a pick and place point; activating an automatic movement control of the robot arm; by the activated automatic movement control, automatically picking the object by the robot arm and returning the robot arm to the visual point from the pick and place point; controlling the image capturing device to capture a second image at the visual point; and forming a differential image by subtracting the second image from the first image, setting an object image according to the differential image and automatically learning an image characteristic of the object; wherein the robot arm is connected to a control device, the vision of the image capturing device is shown on the screen connected to the control device, and the captured first and second images are recorded in the control device; wherein the teaching movement path is the path along which robot arm is pushed to the pick and place point from the visual point and is recorded in the control device.