我们都知道，对于一个给定的球，总有它的外切圆台（不只一个）存在．但反过来，对于一个给定的圆台，却不总有其内切球．为此，本文将介绍抛物线焦点弦的一个应用——有内切球的圆台的判别与构作．定理如果抛物线的焦点弦与其对称轴不垂直，那么这条焦点弦绕其准线旋转一周而 We all know that for a given ball, there is always an circumscribed circle (more than one). But in turn, for a given circular table, it does not always have its own inscribed sphere. For this reason, this paper will introduce an application of the parabolic focal string - the discriminating and constructing of a circular table with an inscribed sphere. Theorem If the focal line of the parabola is not perpendicular to its axis of symmetry, then this focal line rotates around its guideline for one revolution.