Abstract: In order to reduce the influence of advance grouting reinforcement on underground surrounding environment, low pressure slow injection is required for grouting, dense fine sand layer has tight structure and high clay content. As a result, the diffusion radius of existing grouting materials is small under low grouting pressure, and the tunnel face cannot be effectively reinforced. However, the existing grouting materials have a small diffusion radius, making it ineffective to reinforce dense fine sand layers. This study is based on the concept of component activation and particle size optimization, using slag micro powder, ultrafine fly ash, and ultrafine cement as the main components, mixed with an appropriate amount of composite activating coagulant to prepare a new type of advanced reinforcement grouting material. Through indoor experiments, the influence of mix ratio and the amount of composite activating coagulant on the flowability, setting time, and strength change characteristics of the new grouting material slurry were investigated. Using a self-developed small duct grouting full-scale test device, the diffusion characteristics of ordinary cement, ultrafine cement, and the new grouting material slurry were compared and analyzed, revealing the change law of final grouting pressure of the new material under the influence of overlying load and moisture content. The research results indicate that with a water-cement ratio of 1:1, a mass ratio of slag micro powder, ultrafine fly ash, and ultrafine cement of 4:2:4, and a composite coagulant dosage of 4%, the new grouting material slurry exhibits optimal diffusion performance, controllable setting time, and high strength of the stone body. In dense fine sand layers, when using small duct grouting for reinforcement, ordinary cement slurry mainly densifies for diffusion, ultrafine cement slurry mainly densifies and cracks, while the new material diffuses in a permeation-cracking manner with the best diffusion performance and reinforcement characteristics, followed by ultrafine cement, and ordinary cement being the least effective. The new material should maintain low-pressure grouting in the early stage of grouting to prevent excessive grouting, which could cause local sand cracking and reinforcement failure. The final grouting pressure linearly increases with the increase in overlying load and moisture content, with an increase of 50 kPa in overlying load resulting in an approximate 50kPa increase in grouting pressure. A 2% increase in moisture content leads to an increase of around 40kPa in grouting pressure. The research findings have certain guiding significance for grouting theory research and engineering applications.