Abstract: The thermal simulator was used to simulate the coarse grain heat-affected zone (CGHAZ) of a deep-sea pipeline steel, and the microstructure and toughness of the deep-sea pipeline steel were characterized using optical microscope, scanning electron microscope, electron backscattering diffraction, and Charpy impact test. And the effect of different peak temperature in secondary thermal cycle on the microstructure and toughness of the CGHAZ of the deep-sea pipeline steel was studuied. The results show that when the peak temperature of secondary thermal cycle is 800 ℃, the microstructure of CGHAZ of the deep-sea pipeline steel is primarily granular bainite, the massive M-A constituents shows intermittent chain structure at the original austenite grain boundary.The content of high angle grain boundary is low, and the impact absorption energy is 37.8 J, indicating obvious embrittlement. When the temperature reaches 900 ℃, the microstructure mostly consists of polygonal ferrite and quasi-polygonal ferrite, the percentage of high-angle grain boundary is big, and the toughness is the best.