Residual stress is an important factor affecting structural deformation, which seriously reduces the dimensional accuracy, fatigue strength, and safety of structural parts. In this study, a coupled thermal-mechanical finite element model for wire arc additive manufacturing (WAAM) was established based on ABAQUS and Fortran subroutine and the accuracy of the simulation results was verified by experiments. The effects of hammering and hammering temperature on the residual stress in low alloy high strength (HSLA) steel built by the WAAM process were analyzed. The results showed that the effect of reducing the residual stress increased and then decreased with the hammer temperature in the hammer temperature range of this study. The optimal hammer temperature was found to be 800 °C. The maximum longitudinal, transverse, and Z-direction residual stress of the deposited wall after 800 °C interlayer hammering decreased by 20.7, 273.9, and 116.9 MPa, respectively.