Carbon adsorbent materials that were prepared from sunflower straw by a combination of pre-treatment and low-temperature pyrolysis showed better adsorption compared with untreated carbon. Four different pre-treatment agents (steam, alkali (KOH), phosphoric (H3PO4), and salt (ZnCl2)) were analyzed with respect to their effects on the maximum surface area and the micropore area. Samples were measured by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), surface area analysis, and pore size analysis. The surface area, pore volume, and N2-adsorption capacity of the samples were closely correlated with the pre-treating agent. A biochar with a maximum surface area of 877.6 m2/g and a micropore area of 792.8 m2/g was prepared with phosphoric acid (H3PO4) as the pre-treatment agent at a temperature of 400 °C. The main result of the one-stage pre-treatment procedure was the number of micropores. The two-stage, low-temperature pyrolysis procedure focused on the volume of the pores. Carbonized sunflower straw, with pretreated and low-temperature pyrolysis procedures, was judged to be a highly effective and economic method to prepare carbon adsorbents.