The PID parameters of the regulation system of hydropower unit significantly influence ultra-low frequency oscillations. While existing research has delved into the generation mechanism and mitigation strategies, limited attention has been given to the root causes of unreasonable PID parameter tuning and strategies for enhancement. The correlation between system instability at varying hydropower ratios and the single-frequency regulation evaluation of the hydropower unit remains underexplored. This study aims to investigate the interplay among system stability, frequency modulation assessment, and modulation parameter tuning. The experimental results show that regions with lower hydropower ratios can align PID parameters with primary frequency modulation assessment metrics, whereas PID parameters in areas with higher hydropower ratios should be set based on system stability considerations. Ultimately, this paper proposes a PID parameter tuning strategy to enhance responsiveness while upholding system stability and dynamic performance.