本文建立的螺纹中径当量全微分优化逼近模型，考虑了螺距偏差、螺距累积偏差和半角偏差本身及其相互间对中径的综合影响，并用理想螺纹在径向和轴向二维逼近实际螺纹。计算机计算表明，本文提出的全微分优化逼近算法优于常用算法及三点接触式算法。经计算机图形仿真显示算法是正确的。 In this paper, the full-differential equivalent approximation model of thread diameter equivalent was established. The influences of pitch deviation, cumulative pitch deviation, half-width deviation and their mutual influence on the middle diameter were considered. In the radial and axial two-dimensional approximation to the actual thread . Computer calculations show that the proposed algorithm is better than the common algorithm and three-point contact algorithm. Computer graphics simulation shows that the algorithm is correct.