Undesired Resonances on High-Speed Differential Signals Are Studied in This Paper, which is Caused by the Adjacent Narrow Ground Line Without Stitching Vias. Due to Space Limitations in the High-Speed Channel Layouts of Certain Package Applications, the Ground (GND) Line is Often Narrow and Has Insufficient Stitching Vias, Potentially Causing Undesired Resonance in High-Speed Differential Signals. in This Study, These Undesired Resonances Were Investigated using 3D Simulations, revealing that They Can Be Modeled as Parallel-Coupled Half-Wavelength Resonance. the Resonance Frequency of the Parallel-Coupled Half-Wavelength Resonance Structure Can Be Predicted Well using the Formula based on the GND Line Length. Moreover, Three Potential Solutions to Undesired Resonance Are Proposed, Providing a Practical Guide for GND Line Routing in Specific Applications.