Abstract: Mudstone has extremely strong water absorptivity, and its hydration swelling can deteriorate casing’s service environment and reduce its service life. In order to investigate the influences on the service of casing after mudstone is hydrated with different displacement fluids, this paper firstly carried out composition, hydration swelling and mechanic experiments on the mudstone of the target block. Then, the finite element model for the mudstone under different working conditions of hydration swelling was established. Finally, the influences of mudstone’s hydration swelling on casing’s external collapse load and strain were analyzed. It is indicated that the mudstone bedding is developed and the rocks tend to disperse after absorbing water, while polymer chemical solution has weaker influence on mudstone hydration. After mudstone is hydrated, its rock strength decreases. After it is immersed in pure water and polymer chemical solution, the rock strength is reduced by 1/20~1/15 and 1/3~1/2, respectively. Mudstone’s hydration swelling leads to the increase of casing’s external collapse load and stress in hydrated intervals. Compared with pure water injection, chemical solution injection can reduce casing’s external collapse load and stress, which is more beneficial to protect casing and prolong its service life.