In this paper, the northern mountainous area of Fuzhou City which is an ungauged basin has been taken for example to discuss the method of design flood calculation by means of combining the DEM (digital elevation model) and the Xin'anjiang model (three components). The problem of estimating the parameters of the runoff model has been solved by using the parameters of the reference station. In the convergence calculation, the isochrones are obtained by DEM which helps to avoid the cumbersome work of drawing them on the map. With the establishment of the digital elevation model throughout the country, it is practically significant to use it in the hydrological estimation.