The diversion steel branch pipe is large and complex to weld, requiring endurance for long-term water pressure of 6 MPa. There is a need for a high-precision, high-resolution stress measurement technique for comprehensive assessment during water pressure tests. Traditional strain gauges are limited to localized measurements and restricted by waterproofing, application processes, and temperature changes. This study proposes using optical frequency domain reflectometry for stress measurement in the pipe. A standard single-mode fiber sensor is laid along the weld seam for a spatial resolution of 5.8 cm. Stress correlates linearly with water pressure and varies periodically from 0.2 to 0.6 m along the weld seam. Maximum stress reaches 196.2 MPa at the crescent rib weld seam's waist, about 300 MPa at the main conical ring seam, and distributes symmetrically along the intersecting line, with a peak of 352.0 MPa near the waistline and minimal stress at the top and bottom.