The stability of excavation pit in northwest Ji’nan, which locates in alluvial clayey-sand interbedded zone of downstream Yellow River, is greatly affected by soil strata. The pile insertion ratio (defined as the pile length above excavation bottom to the length below excavation bottom) is directly affected by soil strata quality. In this article, the stability of support piles with different insertion ratios are analyzed through theoretical calculation and numerical simulation. The excavation pit at the Shandong Provincial Cultural and Arts Center is taken as engineering background. Research results reveal that parameters of soil strata quality are in linear relations with insertion ratio. By introducing the earth pressure reduction coefficient into the calculation of embedded depth of pile, we found that the optimum insertion ratio is 0.55. According to measured insertion ratio 0.58, the numerical result is verified. The maximum lateral displacement of pile top is 0.05% of the pit depth (H), and maximum settlement 0.04%H, indicating that the excavation structure is stable enough with sufficient safety reserve. Therefore, the insertion ratio in the study case can be reduced to cut project cost and shorten construction period. The research results are of reference value for excavation pit construction in special alluvial areas of the Yellow River.