The acetic acid pretreatment of wood chips has become one of the most promising technologies for biorefinery. This study aimed to provide a quantitative evaluation of the porosity variation during pretreatment based on the fractal dimension methodology. The acacia wood sample was pretreated by acetic acid under different temperatures (140 °C to 170 °C), followed by a three-stage disc-refining at high consistency, and was subsequently characterized by the low-temperature nitrogen adsorption method. The detailed data related to the fractal dimension were obtained by two well-established methods, namely, the Yu Boming (YBM) fractal and Frenkel Halsey Hill (FHH) fractal method. Both the acetic acid pretreatment and disc refining resulted in a higher fractal dimension, which indicated increased irregularity of the pore structure. The mechanism behind the temperature’s effect, where the higher temperature led to a lower fractal dimension, was also explored. Compared to the FHH dimension, the fitting range of the YBM dimension was wider and it had a lower correlation coefficient.