In January 2019，some cases of suspected lymphocystis disease took place in a flounder farm in Shandong Province. Some grey-white cauliflower-like tumours appeared on the surface of infected fish，with the mortality rate of over 30%，which brought serious economic losses to the farm. In order to identify relevant pathogen，some dead fish were collected for histopathological diagnosis and virus detection，then the MCP gene of the virus was sequenced and its genetic variation was analyzed. The results showed that all the histological lesions that could be observed were consistent with the pathological characteristics of lymphocystis disease，and the homology of nucleotides and amino acids of the isolated virus and the reference strains was 78.7% to 99.9% and 86.8% to 100% respectively. According to the phylogenetic analysis of MCP gene，the isolated virus belonged to lymphocystic virus type II，the same as LCDV-C，JF00Kuma，JF03Yoshi，JF03GunNeA，JF，LCDV-K1 and other strains，and it was highly homologous with the isolate of LCDV-C in China. Therefore，it was determined that，upon comprehensive diagnosis，the disease was caused by lymphocystis virus type II. The research provided new data for comprehensive diagnosis and epidemiological investigation of the disease，and some theories and references were available for further molecular biology study related to the virus and preparation of relevant vaccines.