Establishing a benthic index of biotic integrity(B-IBI) for rivers is the premise of the health assessment of their ecosystems. Based on the data of benthos at 12 sampling points in the mainstream of the Ganjiang River during 2018-2019, a total of 24 genera from 3 phyla, 6 classes and 13 families of benthos in the Ganjiang River were identified, of which three dominant species were identified as Bellamya aeruginosa, Branchiura sowerbyi and Parafossarulus eximius. Discrimination of the distribution range, ability analysis and correlation analysis of 18 candidate biological indicators were carried out, and the total number of taxon units, total biomass, relative abundance of crustaceans and mollusks, the complete composition of the biomass of pollution-resistant groups were screened out to establish the index evaluation system. Then the ratio method was used to unify the dimensions of biological parameters, from which the value of B-IBI was obtained by accumulating the scores of each biological parameter. Based on the 25% quantile of the B-IBI value, the health evaluation criteria for the mainstream of the Ganjiang River were finally determined. The results show that four of the 12 sampling points in the mainstream of the Ganjiang River are healthy, one is sub-healthy and one is average. The health status of the remaining six sections is in poor or extremely poor state, and the overall ecosystem of the Ganjiang River mainstream is in a general state of health.