To address the issue of limited application of hexagonal boron nitride due to the low content of active groups on the surface, small-sized hydroxylated hexagonal boron nitride was prepared by using two methods: ball milling and high-temperature heating. The modified hexagonal boron nitride was characterized by infrared spectroscopy, Raman spectroscopy, scanning electron microscope, thermogravimetric analyzer, X-ray diffractometer, X-ray photoelectron spectrometer, thermal conductivity meter and contact angle meter to analyze its structure, crystal properties, morphology, thermal stability, elemental composition, thermal conductivity and hydrophilicity/hydrophobicity. The results indicate that ball milling effectively reduces particle size and surface roughness of hexagonal boron nitride, and also improves its thermal conductivity and hydrophobicity, but it has no effect on its structure and crystal properties. Conversely, the high-temperature heating method not only reduces particle size and achieves hydroxylation but also significantly lowers surface roughness, decreasing the root mean square roughness of hexagonal boron nitride from 1.56 nm to 0.27 nm. However, this method compromises thermal stability, resulting in a 21.14% weight loss (mass fraction) for hydroxylated hexagonal boron nitride at 800 ℃. In addition, high-temperature treatment not only reduces the thermal conductivity of hexagonal boron nitride, but also enables the transformation of hexagonal boron nitride from hydrophobic to hydrophilic materials. A new method for the preparation of small-sized hydroxylated hexagonal boron nitride by ball milling and heat treatment is put forward, providing reference for expanding the application of modified hexagonal boron nitride materials in other fields.