Abstract: During the reaming construction process of the reverse well drilling method, the well reduction diameter and the instability of the surrounding rock are the difficult and difficult problems in the development of the anti-well drilling method. When the reaming passes through a stratum composed mainly of water-softening rocks, the influence of water on the surrounding rock of the well is very prominent. The mudstone with water softening characteristics is taken as the research object. Firstly, the elastic modulus and Poisson's ratio of mudstone under different water immersion time were obtained by water immersion softening experiment, and the quantitative relationship between these two parameters and water immersion time was fitted, and the parameter equation considering softening effect was established. Then, based on the circular axisymmetric elastic solution, the displacement formula under the unsupported state of the reaming well is obtained, and then the parameters considering the softening effect are substituted into this formula. Finally, an analytical model of the displacement and immersion time of the surrounding rock considering the boundary effect is established to evaluate the safety of the wellbore reduction and the instability of the surrounding rock during the reaming construction of the reverse well drilling method. Engineering applications provide a reference.