In this paper, microstructure evolution and mechanical properties of transient liquid phase (TLP) bonding joints of reduced activation ferritic/martensitic (RAFM) steel using Fe–Si–B amorphous foil during homogenization stage were investigated. The results show that, numbers of block-like M23C6 carbides and ~500 nm-size elliptical Fe2B phase are precipitated in the isothermal solidification zone (ISZ), while the larger-size rod-like Cr2B phase with size of ~3 μm is distributed in the diffusion affected zone (DAZ). When the homogenization time extends to 30 min, the amount of Cr2B phase reduces dramatically in the DAZ due to the interdiffusion of the alloying elements. The distribution of Cr atoms along the bonding joint was estimated through a model. The model indicates that the dissolution of the Cr2B phase works dynamically during the homogenization stage. The maximum shear strength was obtained about 558 MPa for bonding 30 min. With extending the homogenization time, the fracture transitions from brittle mode to ductile mode. The precipitation of coarse Cr2B phase is the main factor of the shear failure for short homogenization time. Hence, the bonding quality is able to be significantly improved with adjusting the geometric characteristics of such Cr2B via changing the homogenization time rationally.