Nitrogen-doped hierarchical porous biochar has aroused great interest in energy storage and adsorption applications. In this study, nitrogen-doped hierarchical porous biochar was prepared from corn stalks by in situ pyrolysis with urea as the nitrogen source and NaHCO3 as the green activation agent. The liquid adsorption behavior of biochar was evaluated using phenol as a pollutant probe. The results showed that urea had a synergistic effect on the pore structure development of biochar in the presence of an activation agent. The addition of urea especially promoted the formation of micropores. X-ray photoelectron spectroscopy results showed that nitrogen atoms were successfully introduced into the skeleton of biochar in the form of different nitrogen species, which promote the adsorption of phenol. The phenol adsorption was dominated by micropore filling, and the adsorption capacity was positively correlated with the content of graphitic N. The nitrogen-doped biochar obtained at 700 degrees C with a moderate surface area and a high content of graphite type N exhibited the highest adsorption capacity of phenol. The kinetic analysis also indicates the existence of chemical interactions in the adsorption process.