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Viewshed analysis is extremely computing intensive in geocomputation. Moreover, conventional serial algorithm are often difficult to meet this demand. We propose a parallel viewshed algorithm based on scanning circle which can compute the viewshed area quickly on large scale Digital Elevation Model (DEM). Compared with conventional algorithms, our algorithm computes the viewshed area in an expanded way which can help to avoid the redundant computation. Experiments demonstrate that our algorithm is 20 times faster than the viewshed analysis in ArcGIS 10.0, and can meet the demand of viewshed analysis on large scale DEMs.