Agonistic behavior is a common phenomenon in the culture of Eriocheir sinensis. To explore the adverse effects of loser experience, anxiety-like behavior and feeding behavior of E. sinensis after loser experience were observed, and the levels of serotonin （5-HT） and 5-HT7R gene relative expression were detected. Then, the effects of exogenous 5-HT, fluoxetine, and 5-HT7R agonists and antagonists on the anxiety-like behavior of E. sinensis were observed. The results indicated that the E. sinensis that experienced the fight failure showed anxiety-like behavior, which was accompanied by the increase of plasma 5-HT concentration and the decrease of 5-HT7R gene relative expression level in the thoracic ganglion, and the food intake decreased after the loser experience. Exogenous 5-HT treatment promoted anxiety-like behavior, while fluoxetine treatment alleviated anxiety-like behavior. In addition, treatment with 5-HT7R agonist （AS19） promoted anxiety-like behavior, while treatment with 5-HT7R antagonist （SB-269970） alleviated anxiety-like behavior. These results indicate that the loser experience can lead to anxiety-like behavior and affect anxiety-like behavior through the serotonin pathway. This study revealed the potential negative effects of loser experience and provided theoretical basis for promoting healthy culture of E. sinensis.