A split grouting-reinforced body (SGRB) is the new surrounding rock that forms after split grouting reinforcement in tunnels and underground engineering, and its shear-seepage behavior is one of the critical factors affecting tunnel stability. The effects of seepage pressure, confining pressure, and the roughness of the soil−slurry interface on the shear-seepage characteristics of SGRB specimens were investigated using a modified triaxial shear-seepage coupling test system. The failure mechanism for the SGRB was analyzed taking into account its seepage behavior and mechanical characteristics. The results showed that the seepage process of the SGRB specimens could be divided into four stages according to the seepage velocity, including the waterless, rapid, decelerating, and steady seepage stages, and the corresponding water turbidity in the seepage stages was classified as turbid, mildly turbid, or clear, respectively. The peak shear stress of the soil−slurry interface of the SGRB specimens under seepage was lower than that in the waterless environment, and the peak shear stress decreased from 57.25 kPa (waterless) to 29.37 kPa (a seepage pressure of 0.08 MPa), marking a reduction of 50.74%. The seepage phenomenon of the specimens was related to the ‘seepage-to-confining ratio’, and its critical points in the waterless, seepage, and seepage surge stages were 0.2, 0.4, and 0.6, respectively.