The invention provides a preparation method of a large-size aluminum nitride ceramic substrate. The prepared aluminum nitride ceramic substrate has the advantages of high density, high heat conductivity, surface evenness, small tilting degree and the like. The process is in a one-step molding manner, the production efficiency is improved, and the production cost is reduced. With the adoption of different sintering aid systems, the sintering temperature of aluminum nitride is effectively lowered; and with the adoption of optimized rubber dumping and sintering system, the rubber dumping time andsintering and temperature maintaining time can be greatly decreased. The high-density aluminum nitride ceramics are prepared. With the adoption of a special lamination manner, the residual stress inbiscuits is effectively reduced, and by combining with weight leveling treatment and using a polished boron nitride sagger, the prepared substrate is smooth in surface and small in tilting degree. According to the method disclosed by the invention, one-step molding of the large-size aluminum nitride ceramic substrate is realized, smoothing treatment of the ceramic substrate is not needed, the production efficiency is improved, the production cost is saved, and popularization of the aluminum nitride substrate in more fields can be facilitated.