The invention discloses a high-precision postwelding weld joint tracking and residual stress relieving system. The high-precision postwelding weld joint tracking and residual stress relieving system comprises movable welding equipment and a mobile welding stress relieving robot for relieving welding stress, wherein a weld joint is welded through the movable welding equipment. One synchronous motor is fixedly placed outside the weld seam, and the axis of a rotating shaft of the synchronous motor serves as a reference point; two reflection type distance measurement sensors are fixed to the motor shaft and coaxially rotate with the motor at intervals; reflection pieces are attached to an initial welding point, the welding tip portion of the movable welding equipment and the tail working end of the mobile welding stress relieving robot respectively. The high-precision postwelding weld joint tracking and residual stress relieving system is simple in structure and high in automation degree and postwelding weld joint tracking precision, and is not affected by welding slag or welding defects.