Graphical abstract Highlights ► This study improve the quality of cylindrical BNNTs by milling-assist precursor. ► Turbulent flow is designed to ensure the sufficient reaction during annealing. ► Metastable middle product describes the catalyst growth of BNNTs. ► Catalyst milling processing boosts nitriding as well as oxidation of B powder. ► TGA comparison presents promotion effect of milling-assisted method.