Abstract: The characteristics of tunnel slag rock are changeable and the fineness modulus of final crushed sand fluctuates greatly, which is not conducive to engineering application. Through theoretical analysis and experimental research, the influence of the proportion of each nominal particle size interval on the fineness modulus is studied. Divide 0-5.0 mm sand into 0-2.5 mm and 2.5-5.0 mm intervals, based on the calculation formula for fineness modulus of Construction Sand and combined with measured data, a simplified prediction formula for fineness modulus is obtained. Based on the prediction formula, a control logic is constructed to achieve online allocation and closed-loop control of fineness modulus. The research results show that 0-5.0 mm sand is divided into 0-2.5 mm and 2.5-5.0 mm intervals, and a simplified prediction formula for fineness modulus is derived, the fineness modulus can be quickly and accurately predicted by the ratio of sand between 0-2.5 mm and 2.5-5.0 mm intervals; when the fineness modulus is between 2.6 and 2.8 and the blending coefficient is 2.1, the ratio of m₃ and m₂ is 3.1 to 4.8, and the ratio of m₃ and v is 78.12 to 120.96. The fineness modulus control system based on the simplified prediction formula can realize the fineness modulus stability control, and has a positive effect on the resource utilization of tunnel slag.