To assess the invasion risk of alien marine species, an invasive risk assessment system was constructed based on the characters of introduced marine species and on the analysis of literatures and expert consultancy. The system has 5 primary and 20 secondary indexes, and was constructed using analytic hierarchy process (AHP) method and trigonometrical fuzzy numbers for determining the weight of each index of the system. The invasive risks for 7 typical alien marine species were assessed, subsequently. The results of the system identified the risk grade of cordgrass Spartina alterniflora and White Spot Baculovirus as very high, Mytilopsis sallei, Karenia mikimotoi and Perkinsus spp., as high, Tilapia spp., as medium, and Scophthalmus maximus, as low levels of risk grade. Because traditional “1-9 scale” method is unable to reflect the real thought accurately, which leads to incorrect judgments, an improved scale method based on trigonometrical fuzzy numbers was used in the present assessment system, and the relative weights of both primary and secondary indexes were designed. Thus the assessing results reflect the real thought more accurately. It is believed that the construction of this risk assessment system will provide beneficial suggestions in decision makings for the managements of tackling alien marine species.