本文通过多项式近似导出了一种可变网格的时域有限差分算法，其空间网格能沿任意轴向变化，而且具有高阶离散精度，从而节省了机时，提高了计算精度，并首次应用于微波单片集成电路（ＭＭＩＣ）常用结构的定模分析，具体研究了空气桥、通孔接地对电路性能的影响，其结果与有关文献相一致，验证了算法的正确性和有效性。 In this paper, we derive a finite-difference time-domain finite difference method for variable grids by means of polynomial approximation. The spatial grid can change in any axial direction with high-order discreteness, saving machine time and improving the precision. It is applied to the fixed mode analysis of common structures of microwave monolithic integrated circuits (MMICs). The effects of air bridges and through-hole grounding on circuit performance are studied in detail. The results are in good agreement with those of the literature, which proves the correctness and effectiveness of the proposed algorithm.