In order to enhance the solidification effect of biological enzymes on red sandstone soil more efficiently, the process of preparing test specimens was optimized in the laboratory, which involved screening the red sandstone soil into a target particle size range (P_≤2.36, P_2.36~4.75, P_≥4.75), and directing the biological enzyme to act on the red sandstone soil in the target particle size range. Unconfined compressive strength, static compressive elastic modulus, splitting strength and erosion resistance tests were conducted on biological enzyme cement composite solidified red sandstone soil specimens. In addition, in order to further study the influencing factors of biological enzyme-directed action, SEM tests were carried out on different proportions of soil with particle size below 0.075 mm accounting for the total mass of particles with particle size below 2.36 mm and below. The test results show that a particle size below 2.36 mm is the highly efficient particle size range for biological enzyme cement composite solidified red sandstone soil. Compared with the conventional method of adding biological enzyme to the specimen, the highly efficient particle size of biological enzyme-directed action on red sandstone soil can further improve the unconfined compressive strength, splitting strength and static compression elastic modulus of the specimen, and significantly reduce the quality loss of erosion. The microscopic test results show that the content of clay material has a significant impact on the solidification effect of biological enzyme, and the proportion of particle size below 0.075 mm has a significant impact on the surface morphology of the soil. It is suggested that the biological enzyme be applied to solidify red sandstone soil by improving its construction technology, so that the biological enzyme is only used on soil particles below 2.36 mm.