Abstract: Quinoline Insolubles (QI), as a by-product of coal pitch processing, significantly influence the practical application of pitch-based coatings for silicon-carbon anodes. Using medium-low temperature coal tar pitch as the raw material, this study systematically explores the mechanism by which QI affects the coating performance of silicon-carbon composites through precise regulation of QImass fraction (0~10%). It provides both theoretical guidance and a practical basis for optimizing anode coating processes. Characterization techniques—including X-ray diffraction, Raman spectroscopy, nitrogen adsorption-desorption, scanning electron microscopy, and transmission electron microscopy—were employed to analyze coating morphology and interfacial microstructural evolution, complemented by electrochemical performance tests. The results demonstrate that QI exhibits a dual character of structural regulation and performance balance during coating. When the mass fraction of QI increased from 0 to 10%, the interlayer spacing of the composites expanded from 0.333 2 nm to 0.333 9 nm, and the I_D/I_G value of Raman spectrum increased from 1.02 to 1.11, indicating that QI promoted the formation of amorphous carbon structure. This structural evolution directly influences electrochemical behavior. An appropriate QI mass fraction (2.5%~5.0%) markedly improves cycling stability; however, excessive QI reduces the initial specific capacity from 639.1 mAh/g to 495.2 mAh/g and raises the charge-transfer resistance to 453.1 Ω. Mechanistic analysis reveals that QI acts mainly at three levels. First, its spherical particles (1～20 μm) serve as physical support points that enhance the mechanical strength of the coating; second, QI guides the formation of a gradient carbon-layer structure during carbonization, effectively buffering silicon volume expansion; finally, a suitable QI content stabilizes the electrode-electrolyte interface, whereas excess QI hinders lithium-ion transport. These findings offer new perspectives for the targeted utilization of QI in anode materials, addressing the trade-off between coating integrity and electrochemical performance while proposing a technological pathway for the high-value use of coal pitch by-products.