Si3N4 ceramics are the new package materials for large-power electronics and devices, which are required to be bonded to the high-thermal-and-electrical-conductivity metal, Cu. Among the methods to join Si3N4 and Cu, active metal brazing (AMB) is widely used, but the joint still faces the challenge of interface bonding reliability for harsh services. Here, a novel approach for enhancement of the Si3N4 AMB joint by regulating interfacial products to be a unitary nanograined TiN layer was proposed. To achieve this, the Si3N4/Cu AMB joint was obtained with an Ag–Cu–In–Ti foil whose formation mechanism was investigated, and the detailed morphology of TiN + Ti5Si3 bilayer was observed at brazing temperatures from 740 to 800 °C. Based on the bilayer formation process, adding different Mo contents from 5 to 20 wt % to hinder the formation of Ti5Si3, obtaining a nanograined TiN layer at the interface with hard-to-observed Ti5Si3 particles around. In the seam, Ag and Cu solid solutions and Cu2InTi were refined after adding Mo, but Cu4Ti and Cu3Ti2 were not generated in the seam, only with CuTi distributed dispersedly. Although the fractures both occurred at the Si3N4/filler interface, the joint with Mo additions had higher strength than those with Mo-free. This was because the reduced CTE of the filler alleviated the residual stress during brazing, the refinement in microstructure as a consequence of nucleation-points Mo functioned, and the unitary TiN layer allowed the joints to bear more load particularly. As a result, the maximum strength of joints with Mo additions reached 424 MPa, 115 MPa higher than those brazed with a foil, and significantly more reliable than joints reported by other literature.