Abstract: From October to December 2023, we investigated the resource status of Eriocheir sinensis in the Anqing section of the Yangtze River, for the first time, discovered Polyascus sp. parasitizing E. sinensis in pure freshwater. In order to grasp the current status of Polyascus infection in Chinese mitten crabs during the investigation period, the carapace width, carapace length, body thickness, and maximum width of navel in both parasitized and normal crabs were investigated and counted by using systematic measurements. Comparisons were made between parasitized and non-parasitized groups. Additionally, the hepatopancreatic, gonadal, muscular, and parotid tissues of the parasitized and normal crabs were stained with histologic sections. Molecular biology methods and morphological identification were used to identify the species of the discovered Sacculina. The results showed that a total of 7175 mitten crabs were caught, of which 3666 were males and 3509 were females. Of these, 38 mitten crabs were parasitized, with parasitism rates of 0.80% in females and 0.27% in males. The carapace width, carapace length, thickness body, and body mass of parasitized crabs were all significantly less than those of the normal individuals. The ratio of the maximum width and height of the navel in parasitized male crabs was significantly larger than those in normal male crabs, showing that the navel morphology of parasitized male crabs developed towards that of female crabs. However, the maximum width and height of the navel in parasitized female crabs did not significantly differ from those of normal female crabs. The gonads of parasitized individuals of both sexes were atrophied and degraded. Tissue sections showed that the hepatopancreas and ovaries of parasitized crabs were infiltrated by the interna of Sacculina. The Sacculina found in the natural freshwater were identified as Polyascus sp.. This study investigated the parasitization status of Polyascus sp. from E. sinensis in the Anqing section of the Yangtze River Basin from October to December 2023. It revealed the survival adaptations of Polyascus sp., as well as the physiological morphology and organizational changes of the parasitized E. sinensis in the natural freshwater environment. These findings lay a foundation for understanding the current status of E. sinensis in the Yangtze River Basin and investigating the symbiotic mechanism between Polyascus sp. and the host crabs.