According to our studies on the growth of benthic diatoms in the settlement substrata at abalone hatcheries in both Dongshan county and Zhangpu county, this article addresses the correlations between larval detachment syndrome frequently occurred in recent years and the changes in community structures of benthic diatoms in the settlement substrata for abalone larvae.The results showed that the extent of larval detachment from the plates is related, not only to species compositions of algal films, but to the variation in algal communities as well.The composition of algae on the surfaces of settle substrata, from which larvae suffered severe detachment syndrome, mainly consisted of the species with adhesive growth, poor adhesive strength and strong mobility like Bacillaria paradoxa, which number decreased greatly or disappeared completely (by 55% to 99%) after larval detachment.Algal films producing slight detachment was dominated by the species with strong adhesive strength like Nitzschia panduriformis, Amphora coffeaeformis.Over 95 % of the sample form settlement substrata for slight detachment comprised algal species having strong adhesive strength like Cocconeis scutellum var.parva Grunow , which was characterized by a decrease in density of the weak-adhesive-strength species and an increase in species with strong attaching strength even there were some variations in the total amount of algae.Average ranges of variation of diversity indexes of benthic diatoms from three types of samples were 48.58%, 23.75% and 19.84%, respectively.Changes in homogeneity were 43.43%, 23.75%, 19.84%, respectively.Diversity variation coefficients were 32.39%,13.41% and 2.47%, respectively.M1 the factors were positively related to the extent of detachment of abalone larvae from the plates, and it was revealed that there was certain correlation between detachment syndrome of abalone larvae and instability in community structures of benthic diatoms in their settlement substrata.