The invention discloses a flood inundation calculation method and device suitable for a humid mountainous area and a medium, and the method comprises the steps: S1, carrying out the sub-basin division of a target region, and setting a model calculation range; s2, on the basis of a three-water-source Xinanjiang model framework, a lumped hydrological model of each sub-basin is constructed, through hydrological
similarity analysis and confluence unit line calculation of each sub-basin, hydrological model production and confluence parameters are calibrated, and the calibrated
model parameters are transplanted to each sub-basin; s3, on the basis of an alternative implicit ADI
algorithm, a two-dimensional hydrodynamic model is constructed, the roughness value of the model is optimized, and the roughness value is used for calibrating
model parameters of the two-dimensional hydrodynamic model; and S4,
coupling the three-water-source Xinanjiang model with the two-dimensional hydrodynamic model, and carrying out flooding calculation in the whole process of runoff production, confluence, evolution and overflow. According to the invention, evolution and overflow of flood in a complex
terrain can be accurately simulated, and the flood inundation range, depth and
water flow states at different moments can be accurately calculated.