There is very strong fluid-solid coupling during seeping in porous media. Taking full account of fluid-solid coupling in the COMSOL software for multilateral wells, the distribution characteristics of coupling seepage and stress field are analyzed, and the dangerous section of sidewall is recognized according to calculating the value of fail coefficient by the Coulomb criterion. Through calculating the minimum value of fail coefficient and the volume integration in which fail coefficient is less than - 2.5 MPa, the impact of the build-up rate on sidewall failure is studied. The results showthat the changes of porosity is very small obtained in the simple seepage field by using unique compressibility factor, while the porosity and permeability of the coupling seepage field change remarkably in the vicinity of the bottom. Therefore, fluidsolid coupling seeping is a true reflection of underground seepage. In the stress field, both the maximum valve of Von Mises equivalent stress and the minimum valve of coefficients fail emerge in the side tracking point of the lower lateral, which means that the sidetrack windowis the dangerous section of sidewall. Finally, the conclusion of smallest range of rock failure drilling lower lateral with build-up rate of 15 /30m is obtained, which provides a the oretical basis for the designing and building of the multilateral wells.