Abstract: From June 30 to July 4, 2016, the strongest extreme rainfall hit the Yangtze basin since the flood season.However, in the operation, the prediction of the location of rain belts has a significant error.Therefore, based on the ensemble forecast data from the European Medium-Range Weather Forecast Center (ECMWF), this study analyzed the basic situation of the deterministic and ensemble forecasting using the method of synoptic diagnosis and discussed the causes of forecasting error.The results show that the sub-synoptic scale disturbance along the Meiyu front is dominant in the variability of the Meiyu rain band position, which appears a negative-positive-negative distribution in the region from Huanghuai-Liaodong Peninsula to the Korean Peninsula-eastern Japan.Its strength has an important influence on the change of rain belt position.When the disturbance is strong, it is favorable for the low-level monsoon to extend to the north, and the cold air intensity is weak, thus causing the location of the rain belt to be north, and vice versa.In addition, it is found that the synoptic-scale fluctuation originates from the initial error field in the northeast of Qinghai-Tibetan Plateau by comparing the accurate and northerly members of the ensemble forecast group.Along with the eastward propagation of mid-latitude westerly wind fluctuation, the error moves eastward along the Meiyu front in the middle and lower layers, and increases continuously, finally causing the rain belt in the middle and lower reaches of the Yangtze River to be significantly northerly.