<p>Eccentricity error is a common source of issues in gear systems, leading to problems such as decreased efficiency, increased vibration, noise, and reduced lifespan. Previous research has highlighted that phase adjustment can mitigate vibration, improving system reliability. Therefore, examining the influence of eccentricity error phase on the nonlinear dynamics of gear systems becomes essential. This study proposes a model for time-varying meshing stiffness (TVMS), incorporating eccentricity errors in both the pinion and the gear. The impact of error phase on TVMS is analyzed using both time-domain and frequency-domain curves. An improved dynamic model of a spur gear pair is also developed, considering static transmission error, eccentricity error, and time-varying backlash. The influence of error amplitude on nonlinear dynamics is explored using bifurcation diagrams, frequency spectra, and Poincaré maps. The results show that eccentricity error significantly influences system behavior. As error amplitude increases, the system transitions from periodic to chaotic motion, accompanied by an increase in dynamic load factor. Adjusting the eccentric phase can reduce vibration, with larger amplitudes yielding greater improvements. In some cases, phase adjustment can shift the system from chaotic to periodic motion.</p>