In this study, we will introduce a design method for controlling the zoom curve of the zoom lens by using the position of lens group of effect focal length differential. It can make the zoom curve smoother or easier to manufacture. We give some simple examples, and finally we design a 3X zoom and seven-group zoom lens which a half of field of view from 15.3 degree to 39.8 degree, by using the Code V and this optimization method, and show the result.