Abstract: In this paper,the damage evolution law of low cycle fatigue (LCF) of welded blisk is studied,to which the damage mechanics theory is applied.The LCF damage evolution model is built on the basis of strap in variable thermodynamics,the LCF damage characteristics and the coupled strain equivalence principle with damage.The damage of welded specimens is measured in the strain controlled LCF experiment of titanium alloy (TC4) which is often used in aero engine and the damage evolution model is verified.The parameters in this model have definite physical meaning and are easy to be measured.The research results are of reference value for the life predication of the welded blisk of aero engine.