The invention relates to a method for preparing a high-purity hexagonal boron nitride ceramic by pressureless sintering. The method comprises the following concrete steps: putting commercial hexagonal boron nitride powder as a raw material into a stainless steel mould, performing cold press molding in a bidirectional pressurization manner, and forming in a cold isostatic press; dividing a sintering process into two sections, namely pre-sintering in an air atmosphere in a muffle furnace at first and then performing pressureless sintering in a hot press furnace filled with nitrogen to obtain the high-purity hexagonal boron nitride ceramic. The method is capable of preparing the high-purity hexagonal boron nitride ceramic under the pressureless sintering process conditions; the high-purity hexagonal boron nitride ceramic has the characteristics of being good in high temperature resistance, heat conduction property and thermal shock resistance, easy to be processed into a part with a complicated shape, and the like; moreover, the preparation process is simple, low in production cost and suitable for industrialized production.