Abstract: In many three-dimensional (3D) software, especially in 3D graphics environment system, picking a object on the screen by mouse is a key point in design and implementing these system. The precision of mouse picking decides the performance of picture drawing and disposal. This paper presents a new method to pick up the 3D objects by mouse, which locates the object in 3D space by the ray. With this method, a satisfaction result is shown in the factual project.