Continuous slope monitoring in reservoirs and dams using ground-based synthetic aperture radar interferometry (GB-InSAR) is vulnerable to atmospheric environmental fluctuations. These fluctuations can cause inaccuracies in deformation results derived from interferogram sequences. Moreover, processing large volumes of continuous GB-SAR images is time-consuming, which negatively affects the overall efficiency of GB-InSAR and the feasibility of quasi-real-time deformation analysis applications. To tackle these problems, this paper presents a uniform grid sampling method and interferometric stacking technique based on the phase gradient building on the conventional polynomial atmospheric correction method. A polynomial atmospheric correction method based on downsampled high-quality pixels (HQPs) is then constructed. This method is applied to monitor the deformation of the high slope on the right bank during the construction of the Huangdeng Hydropower Station. Experimental results show that the root mean square error (RMSE) of the binary polynomial model averages 0.039 5 rad, significantly outperforming that of the unitary model and other conventional correction methods. The average RMSE of the proposed method is 0.024 0 rad, comparable to the accuracy before downsampling. However, the overall solution time reduces notably from 2.32 h to 0.80 h. This indicates that the proposed method can significantly improve the efficiency of continuous image atmospheric correction while maintaining modeling accuracy, offering effective technical support for slope safety monitoring.