This　paper　presents　a　new　planning　approach　of　generating　unknown　3-D　models　automatically.　The　new　algorithm　incorporates　the　limit　visual　surfaces　with　the　trend　surfaces　and　selects　the　suitability　of　viewpoints　as　the　NBV　on　scanning　coverage.　The　limit　visual　surfaces　and　trend　surfaces　are　modelled　by　means　of　the　known　boundary　region　data　obtained　from　initial　view.　The　optimal　design　method　is　used　to　obtain　the　maximal　visible　area　of　next　viewpoint　and　correspondding　pose　parameters　in　left　and　right　planning　process　respectively.　And　the　position　that　can　obtain　the　maximal　visible　area　is　defined　as　the　next　best　view　position.　The　reconstrcted　result　of　real　model　showes　that　the　method　is　effective　in　practical　implementation.