Abstract: The causes for errors appear in the process of measuring the specified total elongation strength R_t0.5of a pipeline steel rectangle sample are analyzed, and a tensile test measuring system for pipeline steel rectangle samples is designed and developed.This system consists a spherical hinge photoelectric extensometer composed by a spherical hinge frame structure and an optical grating sensor, combining the high precision of the optical grating sensor and the advantage of the spherical hinge fixing method.The collected signals are transmitted by a data processor to a computer control system, and then processed by the analog-digital converting function and the software calculating function.The measuring errors can be effectively overcame, so the accuracy and the repeatability of testing results are ensured.The system also has the function of recording the entire tensile curve.The requirement of relevant inspection and testing standards for pipeline steel are met.