Abstract: The thermo-elastic-plastic method was used to simulate the T-shaped welding of P91 steel flat plates to study the residual stress distribution characteristics. Subsequently, heat treatment simulation was carried out to study the reduction effect of different types of residual stresses. The calculation results show that the welding equivalent residual stress is concentrated in the weld and heat-affected zone, with a maximum value of 484 MPa, which is distributed asymmetrically along the centerline of the T shape. The residual stress value and influence range in the x direction are bigger than those in the y direction, and it is concentrated in the middle of the transverse weld, showing tensile stress, with a maximum value of 450 MPa. After heat treatment, the equivalent residual stress and the residual stresses of x and y directions decrease obviously. The decrease of the residual stress of the x direction is more obviously.