Objective  To observe the regularity of the changes in neuroendocrine-immune system caused by fatigue due to military operations, and explore the mechanism by which fatigue occurs in military operations. Methods  The subjects were 240 soldiers belonging to a field artillery force. The medical history and physical examination were taken before military operations, and fatigue assessment scale was accomplished as well. The following variables were measured in all the subjects: pituitary-adrenal [adrenocorticotropic hormone (ACTH), cortical hormone (B), 24-h urinary free cortisol (UFC)], pituitary-gonadal [luteinizing hormone (LH), testosterone (T), estradiol (E2)], pituitary-thyroid functions [serum thyroid stimulating hormone (TSH), tetraiodothyronine (TT4), triiodothyronine (TT3), free thyroxine (FT4), and free triiodothyronine (FT3)], and cellular immune parameters (CD3⁺, CD4⁺, CD8⁺, CD4⁺/CD8⁺, B, NK). After 7 d of large-scale and high-intensity field exercises, the above variables were again measured in all the subjects. Results  After high-intensity military operations, the unpleasant feelings were significantly increased, and the compulsive and psychotic scores significantly decreased in the soldiers. In addition, the pituitary-adrenal and pituitary-gonadal hormone levels also decreased (all P<0.001), and the pituitary-thyroid hormone level increased (P<0.05). Moreover, both cellular and humoral immune functions were depressed after high-intensity exercises (P<0.001). Conclusion  The depressed psychological tolerance in soldiers is the psychological factor of fatigue after a high-intensity military operation. The hypocorticoidism and inhibition of hypothalamic-pituitary-gonadal axis are the pathophysiological basis of military operation fatigue. Suppression of immune function is an important reason for an increase of susceptibility to disease after high-intensity military operations.