In this study, corn stalk was pretreated by steam explosion under various processing conditions, and the antioxidant activity of hydrolysates in the post-process washing liquor was analyzed using the 1, 1'-diphenyl-2-picrylhydrazyl (DPPH•) scavenging method. The yield and composition of the hydrolysates obtained under different treatment conditions were also determined; the results indicated that the steam explosion extent had a significant effect on both properties. Under optimized conditions (1.5 MPa, 20 min), the obtained hydrolysate had the highest phenolic compound yield (18.6 mg/g untreated corn stalk) and the highest radical scavenging capacity (IC₅₀=0.24 mg/mL). To confirm the positive effect of phenolic compounds on the antioxidant activity of the hydrolysate, one-step ethanol fractionation was carried out. Due to the enrichment of phenolic compounds, the ethanol-soluble fraction (ESF) exhibited significantly enhanced antioxidant activity with an IC₅₀ value of 0.06 mg/mL, which was close to that of butylated hydroxytoluene (BHT, IC₅₀ = 0.056 mg/mL). Consequently, this work indicated that phenolic compounds have a significant effect on the antioxidant activity of hydrolysate from steam-exploded corn stalk and that simple one-step ethanol fractionation is an effective way to enrich the phenolic compounds in the hydrolysate and improve the antioxidant activity.