Based on the monthly Extended Reconstructed Sea Surface Temperature dataset (ERSST v5) provided by the National Oceanic and Atmospheric Administration (NOAA), this study investigates how the longitudinal position of La Niña affects summer precipitation in Southeast China during its developing phase from 1950 to 2023. The underlying physical mechanisms are also explored. The results show that an eastward shift of the La Niña cold center tends to induce a meridional dipole precipitation anomaly pattern over Southeast China, characterized by increased rainfall over the Yangtze River Basin and decreased rainfall over South China. In contrast, no significant precipitation anomalies are observed in Southeast China when La Niña shifts westward. Further analysis reveals that the eastward-shifted La Niña events are often accompanied by positive Sea Surface Temperature Anomalies (SSTA) in the equatorial western Pacific, which strengthen the zonal SSTA gradient and enhance equatorial easterly wind anomalies. These changes are favorable for the development of an anomalous anticyclonic circulation near the South China Sea, which enhances moisture transport to the Yangtze River Basin while suppressing convection over South China via its subsiding branch. Conversely, westward-shifted La Niña events are featured by a weaker zonal SSTA gradient and negligible atmospheric circulation responses, thus exerting little influence on precipitation in Southeast China. This study highlights the crucial role of La Niña's longitudinal position in modulating summer rainfall patterns over China and provides new physical insights into the ENSO-precipitation relationship. These findings offer important implications for improving flood-season precipitation prediction.