The invention discloses a high-thermal-conductivity silicon nitride ceramic copper-clad plate and a manufacturing method thereof and relates to the field of the copper-clad plate manufacturing technology. The high-thermal-conductivity silicon nitride ceramic copper-clad plate comprises a high-thermal-conductivity silicon nitride ceramic substrate and oxygen-free copper layers; the upper and lower surfaces of the high-thermal-conductivity silicon nitride ceramic substrate are respectively welded with the oxygen-free copper layers; and the welding is a high-temperature welding in a vacuum brazing furnace by using an active metal welding paste. The bending strength of the high-strength and high-thermal-conductivity silicon nitride ceramic substrate is 2-3 times that of an aluminum nitride ceramic substrate, and the cladding of the high-thermal-conductivity silicon nitride ceramic substrate and a thick copper substrate can be realized; the thermal conductivity is 3-4 times that of the aluminum nitride ceramic substrate, so that the heat dissipation performance of the substrate can be improved greatly; compared with a direct copper-cladding process, the active copper-welding process has higher interface bonding strength, less cavities and higher reliability. Therefore, the high-thermal-conductivity silicon nitride ceramic copper-clad plate has the features of high strength, high conductivity and high reliability.