The invention relates to a boron nitride multiphase ceramic, and a sintering method and application thereof. The sintering method comprises the following steps: 1, preparing the powder of a compositesintering aid; 2, preparing composite powder; and 3, subjecting the composite powder to heating, pressurizing and cooling successively under vacuum or in an inert atmosphere to obtain the boron nitride multiphase ceramic. The invention also relates to application of the boron nitride multiphase ceramic prepared by using the method as a side sealing board material. The boron nitride multiphase ceramic prepared by using the method has a density of more than 95%, a small grain size and excellent comprehensive mechanical properties.