Mountainous county area is a critical challenge within China's flood control and disaster management system. Aiming at improving flood prediction accuracy, this study selected Yangshuo County in Guilin as a case and developed a hydrological-hydraulic coupled model for the Tianjia River watershed by HEC-HMS and InfoWorks ICM platforms. By refining and testing the model with floods from 2017 to 2022, the inundation scenarios for 12 different flood frequencies based on flood peak encounters of Tianjia River and Li Rivers were predicted. Statistics analysis were conducted on the area of submerged cultivated land, the number of villages, the submerged area of the old town and the new town under each scenario. The findings indicate: (1) the flooding along the Yulong River banks is influenced prominently by upstream torrential floods. When the flood frequency in the Tianjia River Basin (PTJH) reaches 50a, the submerged farmland area will exceed 8.15 km2, affecting more than 27 villages along the way. (2) The old town is mainly affected by the floods from the Li River. When Li River flood frequency PLJ=20a, the water level of the Li River overflows the embankments, and The West Street will be in a water depth of 1-2m. (3)The new town district demonstrates heightened sensitivity to floods from both the Tianjia River basin and the Li River, When the flood level exceeds either PTJH=10a and PLJ=20a or PTJH=20a and PLJ=10a, the new town area will suffer serious flood disasters. The research results can provide references for flood prediction in mountainous county.