Boron nitride nanocomposites assembled by nanosheets and nanotubes can exert multi-dimensional synergistic toughening and strengthening effects. This material is expected to be a high-efficiency reinforcement additive in advanced structural ceramics. In this study, we designed a universal method for synthesizing gram-scale boron nitride nanocomposites by annealing the precursor containing catalyst in chemical vapor deposition equipment under flowing ammonia, and a combined growth mechanism of surface-diffusion and solid-liquid-solid is proposed. The boron nitride nanosheets were initially formed by a surface-diffusion reaction between boron trioxide and ammonia at 1300 degrees C. At elevated temperatures (1400 degrees C-1500 degrees C), the boron nitride nanotubes grew in-situ from the nanosheets in the presence of catalysts through a solid-liquid-solid mechanism, forming the desired boron nitride nanocomposite.