Abstract: To achieve the effective dynamic sealing of fractures inducing air-leakage around gas extraction borehole and to prevent slurry loss from fractures under negative suction pressure, SCMC-Bent based hydrogel was prepared, which is composed of sodium carboxymethyl cellulose (SCMC), sodium bentonite (Bent) and other additives. Herein, viscosity evolution processes of the SCMC-Bent based hydrogels with different water-material ratios were measured in laboratory. Then, migration and loss properties of the SCMC-Bent based hydrogel in fracture net work of coal grains were studied by the slurry migration experiment under negative suction pressure. Finally, pH evolution of the gel was measured, and micro-structures of SCMC-Bent based hydrogels with different water-material ratios and isolating-standing times were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and X-ray diffraction (XRD), aiming to illuminate the micro mechanism of viscosity evolution. Study results show: ① at a constant temperature, the viscosity of the SCMC-Bent based hydrogel presents the trend of “rise first, and then drop” over time. The higher the water-material ratio, the lower the viscosity of the SCMC-Bent based hydrogel, and the earlier of the time corresponding to the viscosity dropping. ② The higher the water-material ratio, the faster the hydrogel migrating in fracture net, but the earlier the hydrogel loss happening under the negative pressure. Compared to the hydrogels with other water-material ratios, the hydrogel with water-material ratio of 10:1 merely migrates 41 mm after 24 days under negative pressure, indicating that it is more easily to block fractures with air leakage for a long time. ③ The number of montmorillonite layers in the SCMC-Bent based hydrogel increases first and then decreases over time; a new intermediate product, named as SCMC-CXP inserting in the montmorillonite interlayers is existed. With the isolating-standing time lasting, the amount of SCMC-CXP inserted into montmorillonite interlayers increases first and then decreases, and decreases with the increase of water-material ratio. A “sandwich” structure could be formed in the hydrogel, which is composed of the montmorillonite layers and the SCMC-CXP inserted into the montmorillonite interlayers. Further, in the whole SCMC-Bent based hydrogel system, a 3-dimentional net structure forms gradually that is composed of montmorillonite layers, SCMC-CXP inserted into montmorillonite interlayers and other SCMC-CXP outside the montmorillonite interlayers. ④ The 3-dimentional net structure formed is the reason causing the SCMC-Bent based hydrogel viscosity increasing consequently. However, due to the decrease of montmorillonite layer number and the breakage of SCMC-CXP molecules chain over time, the dispersion of the 3-dimentional net structure occurs, inducing the decrease of the viscosity. What’s more, high water-material ratios make the SCMC-CXP insertion amount decrease and weak the interaction of macromolecules, enhancing the water transportation capacity in the hydrogel, and inducing the viscosity becoming lower.