In this study, a flash-heating device was designed and assembled to achieve instantaneous discharge heating of samples under high pressure in the DS 6×14 MN domestic hinged six-anvil large chamber press. By combining large chamber static high pressure and instantaneous discharge heating technologies, the melting state of crystals has been determined by the nucleation and growth characteristics during solidification. The melting behavior of h-BN powder crystal under high pressure was studied by instantaneous discharge heating treatment. Using scanning electron microscopy (SEM), the microstructures of samples obtained by high-pressure instantaneous discharge heating treatment was analyzed in order to assess the melting state of h-BN crystals. It was determined that the melting points of h-BN under 3.4 and 4.3 GPa are (4251±150) K and (4531±200) K, respectively. These results are beneficial for exploring the applications of h-BN and revising the existed temperature-pressure phase diagram of boron nitride.