Abstract: Through such ultra-fining-pretreatment of 40Cr and T10A steels as salt-bath cyclic quenching, high frequency hardening and laser hardening, the microstructure appearance, the superplastic microstructure characteristic and their forming characteristic of isothermal superplastic welded joint of heterogeneous steels are systematically studied by the metallurgical microscope and electron microscope.And the characteristic zone forming model of the joint is established.The experimental results indicate that it occurs obviously the diffusive behavior in the isothermal superplastic welded interface and the characteristic zones of the joint are divided into three zones which are respectively ultra-fined grain zone of interface, transition zone and base metal zone.The microstructure of the welded joint through ultra-fining pretreatment presents the typical superplastic microstructure characteristic in the isothermal superplastic welding process.