According to the fractal theory and with the aid of the technology of GIS,the spatial fractal characteristic of the coastline is studied in this paper Generally speaking,two problems are discussed,one is about how to demarcate the lengths of coastline in different scales,the other is about whether the fractal characteristic exists or not for the different boundaries of tidal flat In a word, some meaningful conclusions are drawn as follows: (1)The fractal characteristic exists for the continental coastline of Jiangsu Province, and its fractal dimension is 1 0696 (2)It conforms to the exponential correlation between different lengths(L) and scales (k) It conforms to the formula lgL(k)=(1 029lgk +2 9544 for Jiangsu Province’s continental coastline (3)The fractal characteristic exists for different boundaries of tidal flat,and the differences of fractal dimensions indicate the differences of comparative intensity of erosion and deposition in the tidal flat (4)It is found that the reciprocity between mainland and sea,as well as the lithology and substantial makeup,may affect the value of coastline’s fractal dimension,except geological tectonics,etc In a word, the formula which is useful to calculate the coastline’s lengths of Jiangsu Province in different scales is given in this paper for the first time,and natural attributes of Jiangsu Province’s coastline which is affected by the reciprocity between mainland and sea are explained by the fractal theory, thus, the cognition about the significance of coastline’s fractal dimension is deepened, so done the cognition about some geographical phenomena