One of the most important stages in the later period of zoom system design is to optimize the cam curve. We can not guarantee the final image surface stable unless the variable parts of the zoom system move along the accurate cam curve. A new method based on ZEMAX is presented in this article. By utilizing ZEMAX Programming Language (ZPL) to create a program, we can obtain the accurate relationship of displacements between the zooming group and the compensating group. Meanwhile, the program can figure out the continuous change of the spot size in the image surface during the whole zooming process, which helps us know the synthetical image quality and provides strong evidence to the reasonableness of the cam curve.