Abstract: In the ordinary cement based slurry reinforced coal body, the self shrinkage of slurry often causes voids at the interface between slurry and coal body.Taking the reinforcement of soft coal with special open off cut in thick coal seams as the engineering background, a method for the anchor grouting of soft coal based on prestressed anchors and self stress groutingwas proposed. Based on the principle analysis of the new prestressed anchor grouting for strengthening coal, the basic properties of ordinary Portland cement and ultra fine Portland cement were comparatively studied to provide a basis for the selection of raw materials for self stress slurry. Through the longitudinal free expansion rate test of the slurry, the optimum content of the expansion agent in the self stress slurry was obtained. The tests of ordinary siliceous slurry and siliceous self stress slurry to reinforce pre fabricated cracked coal body were carried out. The mechanical advantages and failure characteristics of coal body strengthened by the self stress slurry grouting for strengthening special open off cut was compared and analyzed. Through the numerical simulation and on site monitoring, the effectiveness of new prestressed anchor grouting were studied. The results showed that the expansion stress of the slurry stone and the axial restraint force of the high strength grouting anchor can make the broken coal body in a quasi 3D state of stress and realize its strengthening and damage repair. The ultra fine Portland cement slurry can meet the reinforcement requirements of self stress slurry due to its greater fluidity and higher strength. As the content of expansion agent increased, the maximum longitudinal expansion rate of the slurry increased first and then decreased. The optimum content of the expansion agent for the self stress slurry was 10%. The peak strength of coal body strengthened by the self stress slurry under constrained conditions was 4.6% higher than that of coal body strengthened by the ordinary slurry. It showed that the effect of self stress slurry to strengthen fractured coal was better than ordinary slurry. The initial macro cracks of coal samples reinforced by ordinary slurry and self stress slurry both appeared at the slurry coal interface. The final damage form was the whole split ejection damage based on pre fabricated cracks. But their crack development process was different. The number of cracks in the coal samples reinforced by self stress slurry was less than that in the coal samples reinforced by ordinary slurry. In the numerical model, the settlement of the cantilever coal body was larger when it was supported only by anchor cable. After the new prestressed anchor grouting, the amount of sinking was significantly reduced. During the on site monitoring, the stresses of the anchor and cable were relatively stable, and the sinkage of the reinforced coal body was relatively small. It showed that the new prestressed anchor grouting had better effect on strengthening soft and broken coal.