Understanding of the east-Asian summer monsoon has been considerably improved during the last few decades. However, an accurate prediction of monsoon precipitation is still a challenging task. The variability of the east-Asian summer Monsoon (EASM) is usually characterized by very complex features. The reasons for the difficulties may include the strong internal dynamics of the EASM, the strong interaction between the Tropics and extratropics, and the influence of the Western Pacific Subtropical High. The complex land-sea distribution may also increase the difficulties. Therefore, the complex features of the east-Asian summer monsoon require special consideration to improve the realism of the simulation results. A number of studies have examined the effects of horizontal resolution on forecast accuracy, with some recent papers investigating the impact of grid spacing of less than 10 km. In general, the increase of horizontal resolution produces better-defined and more realistic structures of the EASM. A detailed description of the characteristics of the east-Asian summer monsoon and resolution dependency of the simulation will be presented through the high-resolution numerical simulations. The Regional Spectral Model (RSM) is used as a tool. The increase in horizontal resolution can expect that the model bias in the east-Asian monsoon region is reduced.