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Joining of large aspect ratio WC-based cemented carbide and Si3N4-based ceramic at 1650 °C was achieved via spark plasma sintering by using functionally graded material (FGM) as a bonding layer. The influence of FGM layer numbers on the bonding quality of the large aspect ratio WC-FGM-Si3N4composite materials was assessed based on thermal expansion coefficient and residual stress data. The microstructure analysis of the interface between adjacent layers revealed the formation of a structure with grain bidirectional anchoring, which promoted the enhancement of the bonding strength. High-strength joining of the large aspect ratio WC-FGM-Si3N4composite rods with the average shear strength of 210.5 MPa was successfully realised. Furthermore, the rod was ground into an end mill, and the obtained end mill connection strength fulfilled the stringent machining conditions for dry milling of nickel-based superalloys.