Abstract: The covariation characteristics and mechanisms of synoptic-scale wave activities over the Eurasian continent and midsummer (July–August) rainfall in eastern China are evaluated on an interannual scale based on station data, ERA5 reanalysis, and NSIDC sea ice observations. The analysis is performed using bandpass filtering, singular value decomposition, and empirical orthogonal function analysis. Results show that in years with strong synoptic-scale wave activities over West Siberia, rainfall in eastern China presents a “dipole” spatial pattern, with decreased rainfall north of the Yangtze River and improved rainfall to the south. In years with weak synoptic-scale wave activities, rainfall in eastern China displays a “tripole” spatial pattern. This pattern corresponds to enhanced rainfall in South China and North China but diminished rainfall in the Yangtze River basin. Further analyses reveal that in years with active synoptic-scale wave activities, the Northeast Asian high-pressure ridge improves, and its eastern anomalous northeasterly winds restrain the intensity of the East Asian summer monsoon. These phenomena lead to the dipole rainfall pattern. In years with weaker synoptic-scale wave activities, the subtropical high exhibits westward extension and northward advancement, which results in the tripole rainfall pattern. Arctic sea ice is determined as the primary factor affecting the intensity variation in synoptic-scale wave activities over the Eurasian continent in midsummer. Decreased (increased) sea ice from the Kara Sea to the eastern Barents Sea strengthens (weakens) the meridional temperature gradient in the central Eurasian region, which raises (diminishes) the atmospheric baroclinicity in the West Siberian area and improves (reduces) synoptic-scale wave activity. In addition, the strengthening of westerlies over West Siberia in the northern part of the jet stream enables the downstream propagation of upstream synoptic-scale waves, which enhances the synoptic-scale wave activities in the West Siberian region.