1. A method for teaching a robot arm to pick and place an object, comprising: pushing the robot arm until a target appears within a vision of an eye-in-hand vision device; setting an appearance position of the target as a visual point; controlling the eye-in-hand vision device to capture a first image at a visual point; pushing the robot arm to a target position from the visual point; setting the target position as a pick and place point; activating an automatic movement control of the robot arm; automatically picking and placing the object by the robot arm and returns to the visual point from the pick and place point; controlling the eye-in-hand vision 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 the target image according to the differential image and automatically learning image characteristic of the target.