The Spectral Imaging Coronagraph (SICG) is the first ground-based coronagraph independently developed and routinely operated in China. The quality of coronal observation data obtained by the SICG is closely linked to the imaging uniformity of its optical system, making flat-field correction a key step in high-precision solar coronal observations. Based on main observational data acquired between 2024 December and 2025 February, together with supplementary controlled experiments conducted in 2025 June, September, and October, this study systematically analyzes the flat-field variations of the SICG. The results show that the flat-field pattern evolves throughout the day, with significantly larger variations in the outer field of view than in the inner field of view. Further analysis identifies ambient temperature variations, image plane shifts, and dust accumulation on the objective lens as the dominant factors driving this instability. Consequently, this study establishes a scientific foundation for optimizing the SICG flat-field observation strategy. Specifically, it is recommended that flat-field frames should be acquired at local solar noon to achieve optimal calibration performance.