The invention discloses a method for synthesizing high-density hexagonal boron nitride in a high-temperature and self-propagating way. A high-purity and high-density hexagonal boron nitride powder material can be obtained through a material design and a self-propagating high-temperature synthesis technology. The method comprises the following steps of: dosing and mixing materials; forming; drying; self-propagating synthesizing; washing and oven-drying; and detecting. According to the method disclosed by the invention, magnesium powder is used as a main reducing agent, hexagonal boron nitride is used as a revulsive, and high-purity and high-density hexagonal boron nitride powder can be prepared through a self-propagating high-temperature synthetic reaction under a certain pressure. The method disclosed by the invention has the characteristics of simple technology, short procedure and strong continuity, and can meet the industrial production requirements on high production and low cost; and compared with the density of a common product, the density of the high-density hexagonal boron nitride is increased by more than 28%.