Abstract: The microstructure and yield ratio of 34Cr Mo4 steel for high-pressure gas cylinder after water intercritical quenching at 770℃and tempering at different temperatures were studied by OM,SEM,XRD and JMat Pro.The results show that tempering temperature is different,the tempered sorbite+ferrite microstructure is obtained,and the yield ratio is significantly lower compared with the sample after the quenching+tempering process.With the increase of tempering temperatures,the volume of ferrite is basically unchanged,lattice distortion degree is similar,M₃C and M₂（C,N） are re-dissolved,M₂₃C₆ and M₇C₃ grow up,and these changes are most obvious at 630℃.Therefore,the tensile strength of the material decreases gradually,the yield strength first increases and then decreases,and the yield ratio first increases and then decreases,which reach a minimum value of 0.776 at 630℃.