Fine particulate matter (PM_2.5) has recently been associated with the activation of the hypothalamus–pituitary–adrenal (HPA) axis, increasing cardiometabolic risks. However, it is unknown which constituents of PM_2.5 were mainly responsible for these associations. In a longitudinal panel study with 4 repeated measurements among 43 college students in Shanghai, China, we measured serum levels of corticotropin releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and cortisol, as indicators of HPA axis activation. Then, we evaluated the associations of 22 constituents of PM_2.5 with these stress hormones using linear mixed-effect models. During the study period, the average daily concentration of PM_2.5 was 41.1 μg/m³. We found that short-term exposure to PM_2.5 was associated with elevated levels of the 3 stress hormones. We observed that water-soluble inorganic ions, especially nitrate (NO₃⁻) and ammonium, had stronger influences on 3 hormones. Six metallic elements, including Zn, Mn, Cu, Fe, Br, and Cr, had positive but generally instable associations with 3 hormones. The effects of organic carbon and elemental carbon on hormones were generally weak. When correcting for multiple comparisons using false discovery rate, NO₃⁻ was still significantly associated with CRH, but other important associations turned to be insignificant. An interquartile range increase in NO₃⁻ on the previous day were associated with 12.13% increase (95% confidence interval: 4.45%, 20.37%) in CRH. Our findings suggested that water-soluble inorganic constituents of PM_2.5 (especially, NO₃⁻) might have stronger influences on the activation of HPA axis than carbonaceous and elemental components.