Abstract: Microstructure and properties in the coarse grain heat-affected zone (CGHAZ) of HQ130 high strength steel were studied by using the welding thermo-simulation test.Fracture morphology and fine structure in CGHAZ were analy sed by means of SEM、TEM and electron diffraction.Experimental results indicated that the important cause resulted in toughness decrease in CGHAZ is the formation of upper bainite (Bu) microstructure except the coarsing of lath martensite (ML).By controlling welding heat input (E=10~20 kJ/cm) to form mixed microstructure of ML+(10~20)% BL in CGHAZ,preventing to form bainite (Bu+Bg),toughness in the CGHAZ can be maintained at higher level.