The failure of solder joints under thermal cycling is mainly as a result of progressive damage process of solder materials. The objective of this study was to investigate the damage behavior of SnAgCu solder under thermal cycling condition. A damage model was proposed and employed to simulate the thermal cycling behavior of SnAgCu solder. The proposed damage evolution law was based on continuum damage mechanics and an interaction between creep and fatigue. Thermo-mechanical cycling and thermal cycling tests were conducted for model parameter determination. A special bimetallic load frame with single joint-shear solder sample was designed and used to study the damage evolution behavior of SnAgCu solder. The damage variable D=1–R0/R was selected and measured for the single joint-shear solder sample every dozens of cycles during thermal cycling tests to verify the model. The damage evolution law was deduced as power function with thermal cycles and the results show that the experimental damage data can be fitted reasonably well by the relationship as the damage model proposed. The microstructure evolution of SnAgCu solder under thermal cycling was observed by Scanning Electron Microscopy and the damage mechanism was analyzed.