This study analyzes the sources of flow performance errors in liquid-sealed rotary piston water meters，focusing on the impact of internal structural factors (e.g.，O-ring fit in the counter，measuring chamber volume，and internal clearances) and external environmental factors (e.g.，water temperature variations and impurities) on measurement accuracy. The analysis results can provide a reference for performance optimization of piston-type water meters and error control in similar mechanical water meters.