Abstract: The transformation temperature phase diagram of 12% chromium steel is obtained by using Thermal-cal software.Process of welding 12% chromium stainless steel is analysed with the phase transformation temperature scope.The heat-affected zone(HAZ) of the weld joints is observed with optic microscope and scanning electronic microscope, which the microstructure consists of ferrite, martensite, coarse grain heat-affected zone(CGHAZ), fine grain heat-affected zone(FGHAZ) and Ti(C/N).The key elements that affect toughness of CGHAZ are ferritic grain size and martensitic content.The base material has the best impact toughness when grain size of Ti(C/N) reaches 2-5 μm.The lowest ductility-brittle transition temperature in CGHAZ is -22℃ when content of carbon plus nitrogen in base material is about 0.02% and martensitic content in CGHAZ is 40% under some certain welding procedure.