921A high strength steel is widely used as hull material because of its excellent mechanical and corrosion resistance properties. Hyperbaric underwater laser cladding repair is one of the preferred technologies for on-line repair of ships in complex marine environment. In order to explore the distribution law of temperature field, stress field, and strain field of 921A ship steel repaired with hyperbaric underwater laser wire filling cladding, the numerical simulations of atmospheric pressure and 0.3 MPa pressure environment are carried out by SYSWELD software. The results show that: with the change of environmental pressure, the temperature field of the specimen at each stage is spindle shaped, and higher laser power is required to melt the same specimen under high pressure. The equivalent stress and the stress in different directions for both high pressure and atmospheric pressure are less than the yield strength of 921A ship steel. The transverse, longitudinal and axial deformation of the specimen in high pressure environment is slightly larger than that in atmospheric pressure environment, which distributed more clearly in the axial direction.