<p>In this study, corn straw pellets and peanut vine pellets were used as the biomass raw materials, and low-concentration KOH was used as an alkaline modification agent; the effects of the primary pyrolysis temperature and initial morphology of the raw materials on the biochar, as well as the effects of impregnation concentration, secondary pyrolysis temperature, and impregnation time on the physicochemical properties and adsorption performance of modified biochar, were studied. The results showed that low-concentration KOH modification only slightly improved the pore structure and adsorption characteristics of the biochar, and the biochar and modified biochar had similar crystal structures. After modification, the Brunauer–Emmett–Teller specific surface area of the peanut vine biochar increased from 13.38 to 60.53 m²/g, and the iodine value increased from 161.07 to 269.01 mg/g. In addition, the impregnation concentration had the greatest influence on the iodine value. When the iodine value was taken as the target, the optimal conditions for KOH modification of peanut vine biochar were an impregnation concentration of 10 wt%, a secondary pyrolysis temperature of 800 °C, and an impregnation time of 20 min.</p>