More　and　more　robot　tasks　extend　to　unknown　environments.　Extraction　of　the　topological　map　from　the　robot　exploration　results　of　an　unknown　environment　is　of　great　importance　for　the　completion　of　robot　path-planning　tasks.　In　this　research,　a　robot　explores　the　environment　and　builds　a　grid　map　in　the　simulation　experiments;　the　building　of　topological　map　based　on　the　features　of　the　grid　map　is　studied.　A　new　method　is　proposed　that　the　grid　environment　be　classified　to　8　basic　types　of　concave　and　protruding　corners　and　the　8　basic　types　be　defined　by　mathematical　equations.　According　to　the　definition　a　strategy　is　designed　to　divide　the　grid　map　into　s　and　a　topological　map　of　the　environment　is　built.　©　2006　IEEE.