The composite Yb:Y3Al5O12/ Y3Al5O12 crystal was made by thermal bonding method, twelve pieces of composite crystals were made by different technological parameter in manufacturing process. The bonding interface of the composite crystals were observed by optical microscope, transmission spectra of the composite crystals were studied. We get appropriate making technology of the composite crystals from above result. Bonding quality of composite crystals were characterized by the shape of transmission spectra and transmittance of Yb:YAG/YAG composite crystals. All experiments show that the composite YbYAG/ YAG crystal has good bonding quality and integration is feusible.