Abstract: The effects of critical zone accelerated cooling(50 ℃·s-1) starting temperature on microstructure and mechanical properties of X100 large deformation pipeline steel were investigated through thermal simulation test, mechanical property test and microstructure analysis. The results show that (B+F) dual-phase microstructure could be obtained by critical zone accelerated cooling. With the increase of starting cooling temperature, the content of bainite increased and the content of ferrite decreased, which led to the yield strength increase and the plasticity reduce. When the starting cooling temperature was 840 ℃, the X100 pipeline steel had high strength-toughness, as well as yield ratio of 0.69, uniform elongation of 15.5% and strain hardening index of 0.14, these indicated that the steel could accord with the technical requirements of high deformation pipeline steel. Bainite with fine lath and polygonal ferrite with high dislocation density are the key factors offering higher strength-toughness and good deformability of the experimental steel.