In order to study the effect of different thermal cycles on the microstructure and toughness of welding coarse grain heat affected zone (CGHAZ) of deep-sea X70 pipeline steel, the CGHAZ of deep-sea X70 pipeline steel under different heat input (HI) of 10~60kJ/cm was simulated by Gleeble-3800 thermal simulator, and the microstructure and toughness of CGHAZ were characterized by optical microscope, scanning electron microscope and Charpy impact test. The experimental results show that the sample is composed of granular bainite (GB), bainite ferrite (BF) and Mmura A component (MmurA component). When HI increases, the proportion of BF decreases, the proportion of GB increases, the Mmura A component coarsening, the impact absorption energy increases at first, and the excellent combination of BF and GB is obtained in 20kJ/cm. The fracture surface is ductile fracture and the impact absorption energy can reach 173.8J. When HI is greater than 20kJ/cm, the fracture is dissociation fracture, and the impact absorption energy decreases sharply, and the lowest is 18.8J.