A phosphomolybdic acid promoted oxidative pyrolysis was described for preparation of banana pseudostem biochar. The strategy succeeded to introduce oxygen-containing groups onto the biochar surface at low temperature. In addition, pore channels were created, demonstrated by the SEM images of biochar prepared at 200 degrees C. Adsorption experiments showed that the biochar pyrolyzed at 200 degrees C exhibited much better adsorption capability than others pyrolyzed at higher temperatures for methylene blue (MB). The adsorption capability of biochar prepared at 200 degrees C was enhanced from 87.28 to 146.23 mg/g at 45 degrees C with phosphomolybdic acid modification. The enhancement of adsorption capacity could be ascribed to the increase of surface oxygen-containing functional groups and pore channels. In-situ generated carboxylic groups during adsorption and desorption played an important role in removal of MB. Adsorption of MB was mainly attributed to hydrogen bonding, electrostatic interactions and cation exchanges.