This paper focused on how to quickly construct the topological relationship of cadastral data. Based on the space topological information of the cadastral data, a new algorithm of auto-constructing block polygon was given. This algorithm only requires the characteristic information of the graphic in auto-constructing block polygon, so it avoids calculating interior angle, comparing interior angle and coordinates, and considering some special cases. The example and its algorithm analysis have shown that the new algorithm dramatically reduces polygon repetitive search and improves the efficiency of polygon search.