During the double-stage cementing operation in Well SHB-A, early hole-closing failure occurred in the Φ273.1 mm mechanical stage-cementer. According to the actual situation, the physical model of the liquid column distribution in the cementing process is established in this paper, and the formula of differential pressure on the gravity plug is deduced. Here, the pressure difference acting on the gravity piston was calculated as 25.4 MPa according to the fluid parameters of each cementing stage and the liquid column distribution when the injected cement slurry reached 152 m3. Moreover, the pressure-bearing capacity of the stopper was tested. When the pressure acting on the stopper reaches 23.8 MPa, the stopper fails. The experimental data agree well with the theoretical calculation results, which validates our model. It is believed that early hole-closing failure occurring in the stage-cementer in Well SHB-A was because the tool depth and the density difference in the cement slurry and drilling slurry caused the pressure difference acting on the gravity piston to exceed the tool design performance during the double-stage cementing process, and the stage-cementer is not applicable in the well conditions. Based on this failure analysis, the total circumferential angle and the shear front width of the stopper was improved, and the pressure-bearing capacity of the YFZ-A stage-cementer was increased to 30MPa. The improved cementer was applied in Well SHB-B in Xibei Oilfield. [ABSTRACT FROM AUTHOR]