To address the manufacturability challenges of silicon carbide fiber-reinforced silicon carbide (SiCf/SiC) composites and to enhance their industrial applications, this study developed an active unidirectional casting technology to in-situ join SiCf/SiC composites with Ni-based single crystal (SC) CMSX-6 superalloy. The macro- and microstructure, as well as the high-temperature tearing performance of the joint, were investigated. The results demonstrated the successful fabrication of defect-free SiCf/SiC/SC CMSX-6 joints. The in-situ generated CMSX-6 portion displayed a SC structure, with a maximum crystallographic misorientation of approximately 8.34° from the favored [001] orientation of Ni-based SC superalloys. A reaction interlayer composed of a nickel matrix, along with TiC, Cr3C2, and Ni31Si12 phases, formed between the SiCf/SiC and superalloy, exhibiting a crystallographic orientation close to [101]. The maximum shear strength of the joining interface at 800 °C achieved approximately 25 MPa, which exceeds the interlaminar bonding strength of the SiCf/SiC composite. This desirable performance is ascribed to the combined effects of the pinning mechanism, which arises from the infiltration of molten CMSX-6 into SiCf/SiC composite, and the reduction of residual thermal stress afforded by the graded interface formed during joining.