Development of pore structures of activated carbon (AC) from activation of biomass with ZnCl2 relies on content and structure of cellulose/hemicellulose in feedstock. Thermal pretreatment of biomass could induce dehydration and/or aromatization to change structure of cellulose/hemicellulose. This might interfere with evolution of structures of AC, which was investigated herein via thermal pretreatment of willow branch (WB) from 200 to 360 °C and the subsequent activation with ZnCl2 at 550 °C. The results showed that thermal pretreatment at 360 °C (WB-360) could lead to substantial pyrolysis to form biochar with yield of 31.9%, accompanying with nearly complete destruction of cellulose crystals and remarkably enhanced aromatic degree. However, cellulose residual in WB-360 could still be activated to form AC-360 with specific surface area of 1837.9 m2·g−1, which was lower than that in AC from activation of untreated WB (AC-blank, 2077.8 m2·g−1). Nonetheless, the AC-200 from activation of WB-200 had more developed pores (2113.9 m2·g−1) and superior capability for adsorption of phenol, due to increased permeability of ZnCl2 to the largely intact cellulose structure in WB-200. The thermal pretreatment did increase diameters of micropores of AC but reduced overall yield of AC (26.8% for AC-blank versus 18.0% for AC-360), resulting from accelerated cracking but reduced intensity of condensation. In-situ IR characterization of the activation showed that ZnCl2 mainly catalyzed dehydration, dehydrogenation, condensation and aromatization but not cracking, suppressing formation of derivatives of cellulose and lignin in bio-oil. The thermal pretreatment formed phenolic —OH and C=O with higher chemical innerness, which changed reaction network in activation, shifting morphology of fibrous structures in AC-blank to "melting surface" in AC-200 or AC-280.