利用转移矩阵理论，在考虑层间一次反射和转折点处实际相移的基础上，导出了改进的ＷＫＢ公式，数值计算的结果表明本文所得公式的精度远优于传统的ＷＫＢ近似，而且能适用于接近于截止的模式和变化剧烈的折射率分布。 Based on the transfer matrix theory, an improved WKB formula is derived based on the primary phase shift and the actual phase shift at the turning point. The numerical results show that the accuracy of the proposed method is much better than that of the traditional WKB approximation and can be applied to Close to the cut-off mode and a sharply varying refractive index profile.