本文以电场的三大定理、连续超张弛迭代算法、拉格朗日插值算法等原理知识作为理论基础，以 C 语言为编程工具，将由根据以上物理和数学原理所推导出来的模型转化为可以计算像管内电势的 CAD 程序软件。所编写的程序可以在给定像管参数和电极电位的情况下，计算圆柱形像管内任意一点的电位大小，并绘制出像管内的等势线；拉格朗日插值得到对称轴上任一点的电位值。 -In this paper, we base on the three theorems of the electric field, continuous ultra-relaxation iterative algorithm, and Lagrange interpolation algorithm. We use the C language as a programming tools, and transform the model derived above into a CAD software to calculate the electric potential in the image tube. In case the image tube parameters and electrode potential are known, the program written can get the potential of every point in the image tube, and plot equipotential lines inside the image tube. It can also get the potential of every point on the axis of symmetry by Lagrange interpolation.