The integrity of wellbore structures, which is the basis of oil and gas production in soft mudstone, is significantly affected by drilling and cementing.A coupling wellbore structural model of casing-cement sheath-mudstone formation was proposed in this study to study the mechanism of wellbore structural instability during drilling and cementing.Plastic softening and elastic modulus hardening models were employed to describe the constitutive relationship of soft mudstone and cement sheath, respectively.The plastic damage of near-wellbore formation and cement sheath in the physical processes, namely, drilling, casing down, cement slurry injection, and hardening, was numerically analyzed with different fluid pressures and boundary loads on the wellbore.Results demonstrate that drilling and seepage-stress coupling are the important mechanisms of wellbore structural instability in soft mudstone.Furthermore, cementing and cement hardening can further promote the plastic damage of the soft mudstone near the wellbore, with a 10% increase range.The tensile hoop and the radial compressive stresses during cementing, which lead to the cement sheath's fracture leakage, are the main reasons of cement sheath failure.Meanwhile, a cement slurry system with low elastic modulus and high Poisson's ratio can maintain the integrity of the wellbore structure in subsequent production and operation.The proposed method provides a certain reference for optimizing construction parameters and cement slurry systems during drilling and cementing.