Alien Species Invasion, the important part of research on Global Change, has become the focus of protection and administration of bio diversity throughout the world. Recently adequate discussion and research have been given on the negative influence of alien species on ecosystem,with the ignorance of their positive parts. How to objectively access the action and possible influence is the precondition and indemnification of sidestepping the risk brought from alien species. Based on the concept of alien species, native species, alien species invasion and its harm, the positive and negative influence of alien species and population amplification on diversity of species are expatiated systematically. The possible influence path and its mechanism are summarized objectively, includ ing: (1) competition and exclusion, (2) hybridization, (3)interrupting or changing the relationship between food chain, (4) bringing virus, and so on. It is drawn that the influence path and mechanism arise from synthesis of numerous factors. The reason that alien species bring harm and loss to human lies in that man lack correlative knowledge, preventative means and consciousness, an d effective management. To sidestep the risk arisen from alien species, we should avoid introducing alien species intentionally and plan it deliberately and systematically after taking sufficient safe measures. At present the knowledge on alien species invasion is factionary, partial and superficial. The urgent work at present is to study biological and ecological characteristics of alien species and the ecological mechanism of invasion. It is suggested that we should strengthen such work as background of investigation, present assessment, the invasion path, factors affecting invasion, the ecological process and effect of alien species invasion and evolutionary consequence, and so on.