The invention relates to a method for preparing boron nitride nano-tube. Boron-source catalyst precursor reacts in nitrogen, ammonia gas or mixed gas of ammonia gas and hydrogen, and crude products are separated and purified; the boron-source catalyst precursor is formed by the mixture of boron-containing compound, magnesium powder, alkaline-earth metal compound and transition-metal catalyst; theelement mol ratio of B: Mg: M1: M2 is 1: 1.65-1.70: 0.165- 0.182: 0.01- 0.06, wherein, the M1 is alkaline earth metal element in the alkaline-earth metal compound, and the M2 is transition-metal element in the transition-metal catalyst. The invention has the advantage that as the boron-source catalyst precursor is adopted, mild reaction condition can be realized, and the preparation process is simpler; the prepared boron nitride nano-tube has even thickness as well as more than 85% of yield coefficient and more than 80wt.% of purity of the boron nitride nano-tube compared with raw material boron element.