Objective Nuclear factor of activated T cells 5 （NFAT5） can promote T cell development and activation， and exert immunoregulatory function by inducing different target genes. This study aims to investigate the relationship between NFAT5 and thymoma with myasthenia gravis （MG） and the underlying mechanisms.Methods Immunohistochemistry （IHC） was used to determine whether the protein expression of NFAT5 in thymoma was associated with the presence of MG. The microRNA （miRNA） and downstream target gene of NFAT5 were predicted using bioinformatic tools， and were validated in clinicopathological samples using RT-qPCR and IHC.Results A total of 56 paraffin-embedded thymoma samples were examined for NFAT5 by IHC. The positive rate of NFAT5 was significantly higher in thymomas without MG （group T， 81.25%， 26/32） than in thymomas with MG （group MT， 45.83%， 11/24） （P=0.006）. Bioinformatic analysis predicted that miRNA-17-5p targeted and regulated NFAT5， and the downstream gene of NFAT5 was CD4. Ten paraffin-embedded thymoma samples were examined for miRNA-17-5p expression by RT-qPCR， with 5 from group T and 5 from group MT. The expression of microRNA-17-5p in group T was significantly lower than that in group MT （P=0.033）. Among the 56 thymoma samples， the positive rate of CD4 was significantly higher in group T （96.87%， 31/32） than in group MT （79.16%， 19/24） （P=0.034）. There was a positive correlation between the expression of NFAT5 and CD4 for both group T （P=0.017） and group MT （P=0.020）.Conclusions The higher expression of NFAT5 in thymoma without MG than in thymoma with MG indicates that NFAT5 plays an inhibitory role in the development of MG in thymoma. MiRNA-17-5p may target and inhibit NFAT5 in the microenvironment of thymoma， which suppresses CD4 on the surface of CD4⁺ T cells， reduces the response of CD4⁺ T cells， and thus promotes the development of MG.