Urban landscape is unstable,open outward,fragmented and fragile,which is apt to the invasion of alien species.Alien grass,rodent beasts and infectious insects constitute high percentage of urban plants and wild animals. In urban breedings and agriculture,many alien species are imported,especially the pets like cats and dogs.Alien invasive species can bring infectious diseases,constituting a huge threat to people’s health.They reduce the aesthetic value of urban landscape and handicap the healthy development of tourism.Besides,they can destroy the urban infrastructure and induce many disasters,threatening urban ecological safety.Based on the analysis of the characteristics of the areas which are fragile to the invasion of alien species,the theory of safety pattern of landscape ecology is applied to make a spatial strategy to defend alien biological invasion.