The shearing force's lag phenomenon of T beam's flange slab should be considered by using moment-curvature method analysis and abbreviated formula calculating. This paper presented the variational method which can analysis the shearing force's lag effect effectively. 在用弯矩曲率法分析或用简化公式进行计算时必须考虑T梁翼缘混凝土板的剪力滞后现象,用变分法能有效地分析其剪力滞效应。