In this study, the impact of Incremental Analysis Update (IAU) initialization on the forecasts of a squall line event that occurred in southern China on 9-10 April 2016 was investigated. The ERA5 global reanalysis was incorporated into the high-resolution regional model WRF through the IAU method. Results show that: (1) IAU initialization can effectively reduce the imbalance caused by the introduction of low-resolution global reanalysis field into high-resolution regional model and retain the microphysical information in the background field of regional model, reducing the model spin-up time. (2) Compared with the cold start run initialized directly by the ERA5, the linear structure and precipitation distribution of the squall line system using IAU initialization are closer to the observations. (3) Further diagnostics indicate that the improvement of the squall line forecasts using IAU initialization is mainly related to the faster development of cold pool caused by retaining the microphysical information in the background field of regional model and more favorable stratification conditions for the development of squall line corrected by the IAU increment.