Understanding the mechanism of KOH activation in the preparation of activated carbon (AC) enables more efficient utilization of biomass. In this study, brewer’s spent grains (BSGs) were carbonized at 500 °C to produce biochar (BC), followed by KOH activation under different activation conditions. The gas and solid products generated during the activation process were analyzed by gas chromatography (GC), X-ray diffraction (XRD), Raman analysis, a surface area and pore size analyzer, and X-ray photoelectron spectroscopy (XPS). The results show that increasing the KOH/BC ratio or the activation temperature could both promote gas production. XPS results indicated that the activator reacted first with -COOH and then with -OH of ACs, with AC_5-700 having the highest C-OH content (50.04%). As the KOH/BC ratio increased, more aromatic structures were destroyed, and the porosity of ACs was significantly enhanced, with AC_7-700 having the highest Brunauer–Emmett–Teller (BET) specific surface area (S_BET) (2997.69 m²/g). At low temperatures, KOH reacted with the active groups of BC and carbon at the edge of the aromatic structure. At high temperatures, the activator (KOH, K₂O, and K₂CO₃) reacted with carbon in the aromatic structure to generate a large number of pores on ACs and expand them. ACs exhibited more pores with higher KOH addition, and a higher activation temperature did not generate more new pores, but expanded the pores more significantly than high KOH addition.