Surface elevation is the basic data for geo-science.It is difficult to retrieve tidal-flats' elevation from a single Remote Sensing(RS)image because of the complicated sediment dynamical environment and huge spatial difference in tidal-fiats' moisture content.A Digital Elevation Model(DEM)construction method for inconstant inter-tidal zone based on high tempo-resolution MODIS data set in a short period is proposed in a case study on the Dongsha Sandbank of the Jiangsu Radial Tidal Sand-ridges.In the present study,a hatch-preprocessing method based on image partition to handle massive MODIS 1B images is developed and applied to 8163,scenes of MODIS images.The dataset of short-period and multi-temporal MODIS images for inter-tidal flats' DEM inversion is selected and the usability of MODIS dataset is analyzed.Shorelines of the Dongsha Sandbank are extracted by use of batch supervised classification.In accord with tidallevel forecasted by the Chenjiawu Tidal Gauge Station at the overpass moment of each RS image,DEMs of inter-tidal flats in January and summer(Jul,Aug and Sept),2003 are built under ArcGIS9.2.Studies show that:(1)The dataset of short-duration and multi-phase MODIS images can be used to retrieve the historical DEM of tidal-flats at changeful tidal flats.(2)Analysis on usability of MODIS images front Aqua and Terra indicates that there are more usable and highquality MODIS images in spring,autumn and winter,hat less in summer.Therefore,the period for building inter-tidal flats' DEM is suggested to be one month in spring,autumn and winter and three months in summer.