This paper adopts large diameter drill hole cast-in-place Pile of one Highway Bridge as the study case, using the pile calculatingly program, GPILE, that is a part of the common program software system of finite element method, SAP84, to analysis the pile group effect of large diameter drill hole cast-in-place Pile with different coefficients of pile group effect. This paper suggests using the ratio of the volume of pile and soil in the ambient range of pile group to express the new reduced coefficient and introducing the p-y curves of pile group after reduced as horizontal interaction model. Under these conditions, pile group nonlinearly effect of large diameter drill hole cast-in-place Pile is analyzed. At the same time, some different coefficients of pile group effects were calculated using this method. Compared these results, the feasibility and conciseness of reduced coefficient are testified.