In order to achieve the resource utilization of corn stover fiber waste and reduce environmental pollution caused by incineration, a 5% sodium hydroxide (NaOH) solution was used to modify and enhance the fibers. The effect of fiber modification on the setting law and compressive strength performance of alkali slag adhesive sand was studied and compared with unmodified fibers. The results show that with the gradual increase of modified fiber content, the initial and final setting times of the specimens gradually shorten, the flowability slightly decreases, and the compressive and flexural strength show a significant increase followed by a slow decrease. As the content of unmodified fibers gradually increases, the initial and final setting times of the specimens first increase and then decrease, the flowability significantly decreases, the compressive strength significantly decreases, and the flexural strength slightly increases. After fiber modification, surface impurities are removed, roughness increases, and the bonding ability between the fiber and alkali activated matrix is enhanced, which is the main reason for the significant improvement and reduction of mechanical properties and their impact on fluidity.