Abstract: In recent years, to liberate coal resources from high pressure limestone water on the coal seam floor, North China Coalfields have generally adopted surface directional drilling technology to carry out regional grouting reinforcement and transformation (commonly known as “floor regional treatment”) on the thin-layer limestone of the Taiyuan Formation in order to comprehensively seal karst cracks in limestone and block vertical guide water channels. In this technology, the design of the spacing between “horizontal branching holes” closely related to the diffusion range (radius) of the slurry has been widely studied by academia and industry. There is a large amount of grouting work in the bottom plate area of the Anhui North mining area, especially in the mining of deep resources, which will cost billions of yuan. It is necessary to verify the true data of the diffusion range of the grout. Therefore, based on the Hengyuan Coal Mine in the northern Anhui mining area as the research base, relying on the II63 mining area floor area treatment project, the slurry diffusion range tracing test was designed and implemented. The fluorescent agent (tracer) was added to the horizontal branch hole (Z8-7) in the middle, and the rock debris samples were taken from the horizontal branch holes (Z8-6, Z8-8) and cross branch detection holes (Z8JC) on both sides to identify fluorescent cement and obtain the diffusion range of the slurry. Furthermore, based on the analysis of the influencing factors of slurry diffusion, a formula for calculating the diffusion range of slurry in the grouting treatment of the bottom plate area of the Hengyuan Coal Mine was constructed. The results show that: ① Based on the analysis of on-site and indoor identification results of rock debris, the diffusion range of grouting slurry under the coal seam floor area of the Hengyuan Coal Mine II63 mining area was 38.3−44.0 m, and the cement distribution was dense within the diffusion range of horizontal branch hole slurry within 30 meters. The grouting effect was the best in this area. ② Through a rapid identification of on-site rock cuttings and precise identification of indoor rock cuttings, the diffusion range of the slurry obtained was basically consistent, proving the effectiveness of fluorescence tracing of the diffusion range of the slurry. ③ Through comparative analysis, it was believed that under actual grouting conditions such as calculation parameters and boundary constraints, the theoretical calculation and numerical simulation results of the slurry diffusion range were close to the measured results of on-site tracing experiments. ④ Using the data from water pressure tests, grouting parameters, drilling structures, and hydrogeological responses during the tracer test process, taking into account the factors such as gravity, structure, and groundwater runoff, and using SPSS nonlinear fitting software, a formula for calculating the diffusion range of grouting slurry in the bottom plate area of the Hengyuan Coal Mine II63 mining area was obtained. ⑤ Based on the actual geological and hydrogeological conditions of the injection layer in the Hengyuan Coal Mine, using the fitted slurry diffusion range calculation formula, the slurry diffusion range of Z8 site in the II63 mining area was obtained to be 37.8−42.9 m, which was similar to the measured results of the slurry diffusion range tracer test. The calculation formula could be promoted and applied under similar conditions. The on-site tracing engineering test of the diffusion range of grouting slurry in the coal mine floor area not only obtained real data on the diffusion range of slurry, but also clarified the inherent relationship between slurry diffusion and various geological and hydrogeological factors. The diffusion mechanism of grouting slurry for ultra deep and ultra long directional drilling was revealed, and a formula for calculating the diffusion range of slurry was constructed, providing a reference basis for the reasonable design of horizontal branch hole spacing in bottom plate area treatment projects under similar conditions.