Abstract: P91 steel is a typical martensitic heat-resistant steel, widely used in high-temperature and high-pressure components in power plant boilers, petrochemical industry, and other fields. Residual stresses generated during the welding process can superimpose with the working stresses during service, accelerating creep, fatigue, and corrosion, leading to early failure of the welding joint. The groove angle of the joint has a significant impact on the quality of the welding joint. This paper uses a blind hole method to measure the distribution of residual stresses in multi-layer and multi-pass welding joints of P91 steel plates, and uses the finite element method to study the effect of the joint groove angle on the residual stresses of multi-layer and multi-pass welding joints of P91 steel plates. The results show that an increase in the joint groove angle leads to an increase in weld width and the number of welding passes, with the peak lateral residual tensile and compressive stresses on the lower surface both decreasing. For the longitudinal residual stress, the peak tensile stress on the upper surface decreases from 604MPa to 544MPa, a reduction of about 11%. The peak compressive stress increases from 209MPa to 261MPa, an increase of about 24.9%. The peak tensile stress in the heat-affected zone on the lower surface increases by about 10MPa. For the transverse residual stress, the peak tensile stress on the upper surface of the weld plate increases from 545MPa to 648MPa, an increase of about 18.9%. The peak compressive stress decreases from 176MPa to 134MPa, a reduction of about 23.9%. The peak tensile stress on the lower surface of the weld plate decreases from 325MPa to 240MPa, a reduction of about 26.2%. The peak compressive stress decreases from 102MPa to 37MPa, a reduction of about 63.7%.