The 40Cr steel surface waiting for being welded was ultra-fined through laser quenching, and isothermal superplastic solid state welding was conducted between the 40Cr steel and Cr12MoV steel. The mechanical properties of the joint were evaluated by tensile testing and microhardness of the joint and thereby. The microstructure of 40Cr steel’s quenching layer and the joint interface zone was observed and analyzed using scan electron microscope. The experimental results show that ultra-fine microstructure is achieved in the 40Cr steel’s surface waiting for being welded. The superplastic solid state welding can be carried out under welding parameters including welding temperature of 800 , pressure welding time of 4 min and initial strain rate of 1.5×10-4 S-1. The tensile strength of joint achieves 81.94% that of 40Cr steel. The microhardness of transition region in the Cr12MoV side is higher than other region. On the contrary, the microhardness in the 40Cr side is lower than other region. The new M7C3 type carbide is checked in the Cr12MoV side. In welding process, the carbon atom has diffused from 40Cr steel side to Cr12MoV steel side. Therefore, isothermal superplastic solid state welding with higher quality can be realized between Cr12MoV steel and the 40Cr steel after microstructure ultra-fining through laser quenching.