The glass-to-metal seals are usually used in the solar thermal power (STP) and electronic devices. However, the requirement of mechanical properties in the STP is much higher than that of electronic devices, because the glass-to-metal joints used in the STP need to have anti-fatigue performance in adition to higher static tensile strength. Under the repeated fluctuating loads, damage and failures of glass-to-metal seals in the STP often lead to serious consequences. Therefore, analysis of damage evolution and fracture behavior of glass-to-metal diffusion welded joints was performed in this paper. Firstly, the finite element (FE) model of glass-to-metal welded joints was established in accordance with the STP welded structures. And damage simulation was carried out by the FE software ABAQUS. Also, the work illustrates the modeling of damage in terms of traction versus separation to simulate crack propagation and introduces the use of traction-separation law as a damage initiation and evolution criteria. The microgram of damage distribution in the glass side near the interface could be characterized by Scanning Electron Microscope (SEM), which was compared with predictions obtained by finite element method (FEM) analysis. As result, the damage criteria on the lap joints in conjunction with FM analysis were used to optimize the glass-to-metal diffusion welding technology. The above results provide the basis of design against damage and reliable estimation of glass-to-metal seals.