Abstract: During the grouting process of micro-fractured rock mass,the infiltration effect of cement slurry has a signifi- cant influence on the grouting effect. The self-designed micro-fracture grouting visualization test system was developed, which consists of grouting system,micro-fracture model and monitoring system. Monitoring system are further divided into the microscopic monitoring system and the pressure and flow monitoring system. After the cement slurry enters the micro-fracture model,the microscopic monitoring system is used to observe the infiltration effect at the entrance of the fracture in real time. The pressure and flow monitoring system is used to automatically record the grouting pressure and cumulative flow during the grouting test. The self-designed micro-fracture grouting visualization test system is used to conduct experimental research on the cement slurry infiltration process in micro-fissure. Using three types of cement slurry materials (ultrafine cement I,ultrafine cement II,and ordinary cement),under the conditions of grouting pres- sure of 2. 0 MPa and water-cement ratio of 1. 0,the slurry infiltration effect of different cement particle sizes under dif- ferent fracture openings was studied. The results show that when the fracture opening is small,a complete semi-circular arch filter cake is formed at the entrance of the fracture. With the increase of the fracture opening,intermittent filter cakes are formed at the entrance of the fracture. When the fracture opening is increased to the extent that no infiltration occurs,only a small amount of cement particle attachment remains at the fracture entrance. The minimum injectable fracture opening size bmin and the minimum non-infiltration fracture opening size bcrit of the three cements were ob- tained. The bmin and bcrit of the ultrafine cement I were 80 μm and 280 μm,respectively. The bmin and bcrit of ultrafine cement II were 100 μm and 300 μm,respectively. The bmin and bcrit of ordinary cement were 140 μm and 310 μm,re- spectively. The reduction of cement particle size has a greater impact on the minimum injectable fracture opening,but has a smaller effect on the minimum non-infiltration fracture opening. Combined with the percolation trend k value,as the particle size of the cement decreases,the corresponding kmin and kcrit values increase accordingly,and the cement slurry is more susceptible to percolation at the fracture entrance. This is because the ultrafine cement has a larger spe- cific surface area and is prone to agglomeration between cement particles.