The olive flounder (Paralichthys olivaceus) occupies an important position in the mariculture. It has proved as an useful model for study of caudal neurosecretory system (CNSS). The CNSS is a unique neurosecretory structure of fish that may be involved in thermoregulation and reproduction. Previous studies suggest that follicle-stimulating hormone (FSH) signaling is involved in the temperature-dependent sex determination and reproduction in olive flounder. However, a direct effect of FSH on Dahlgren cells remains underexplored. Here, we examined the electrophysiological response of Dahlgren cell population of the CNSS to FSH and the transcription of related key genes of CNSS. In addition, the gonadosomatic index (GSI, %), the expression level of fshβ in pituitary, and FSH receptor gene (fshr), corticotropin-releasing hormone gene (crh), urotensin Ⅰ gene (UⅠ) and urotensin Ⅱ gene (UⅡ) in CNSS during the breeding season were also analyzed. We found that FSH increased overall discharge frequency and may have changed the discharge pattern from silent to burst discharge in a subpopulation of Dahlgren cells. The effect of FSH on Dahlgren cells discharge activity was blocked by the FSH-receptor binding inhibitor (FRBI-8). The mRNA levels of fshr did not differ between FSH treatment and control group. While treatment with FSH stimulated the expression of L-type voltage-dependent calcium channel (L-Ca²⁺). In addition, a positive correlation was observed between the fshβ mRNA levels and GSI or UⅡ mRNA levels during the breeding season. These findings proved for the first time that FSH could act as a modulator within the CNSS and enhanced our understanding of the physiological role of the CNSS in reproduction and seasonal adaptation.