Activated carbon (AC) with high specific surface area was prepared by using bifunctional HPO agent, which led to dehydrating and activation effects through hydrothermal pretreatment and subsequent pyrolysis process. N adsorption and desorption isotherms of AC showed a high BET surface area of 2434 m g and a total volume of pores (V) of 2.0447 m g for AC. The morphology and the chemical components of hydrochar and AC were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy, which indicated that HPO was benefitting for the formation of porous structure of AC. Subsequently, the effect of HPO in hydrothermal pretreatment and activation process was investigated by comparative experiments. The removal and adsorption of methylene blue (MB) dye with different concentrations onto the AC were studied. The monolayer equilibrium adsorption capacity was 644 mg g, showing that AC has good adsorption qualities for methylene blue (MB). The adsorption balance data of MB on AC was best fitted to the Redlich-Peterson model. The adsorption kinetic data fit better to the pseudo-first-order model at low MB concentration, and the pseudo-second-order and Elovich models fit better when the MB concentration was rising.