To further study the interaction between diatoms and marine invertebrates in natural biofilm, and to explore the diatom communities in natural biofilms of different ages in the Gouqi Island waters and their effects on the attachment of Mytilus coruscus. Natural biofilms at 7, 14, 21 and 28 days of age were formed at the sea area of Gouqi Island (122°46"E; 30° 43"N). The dry weight, bacterial density, diatom density, chlorophyll a concentration, and diversity of diatom community were analyzed. The inducing activity of biofilms on the settlement of M. coruscus was carried out. The results showed that the diatom density, chlorophyll-a concentration, dry weight, and bacterial density were positively correlated with the age of the biofilm(P<0.05). 21 d and 28 d were high day old biofilm, they had higher entrapment capacity and the diatom density reached its maximum at 21 d. Analysis by 18S rRNA sequencing revealed that about 89% of the diatoms in the biofilms used in this study had a positive effect on the settlement of M. coruscus. Lumiphores, Odontella, and Achnanthes were the dominant diatoms in the high-day-old biofilm.The species of diatoms and their relative abundance in the biofilm were correlated with the induction ability of the biofilm. The results of this study provide a theoretical basis for the study of the relationship between the diversity of the diatom community in the biofilm and the settlement of invertebrates, it is of great theoretical significance for the potential application of the high -efficiency cultivation of diatom and its biofilms in the shell mussels such as M. coruscus .