Abstract: Stocking space significantly influences the welfare of cultured animals and modulates their physiological homeostasis. This study investigated the effects of stocking space on stress response and growth of grass carp (Ctenopharyngodon idella) cultured in a recirculating aquaculture system. Grass carp were cultured at a density of 20 kg/m³ in two different stocking spaces: 0.47 m³ (small stocking space, SSS) and 2.98 m³ (large stocking space, LSS) for 10 weeks. LSS group exhibited significant enhancements in body weight, weight gain rate, and specific growth rate. Conversely, fish in the SSS group showed a reduced feed conversion ratio, indicating compromised growth performance. Histological analysis demonstrated that the LSS group had significantly enlarged muscular fiber diameter, accompanied by a decrease in muscular fiber density. Molecular analyses revealed that a larger stocking space significantly upregulated the relative expression of ghrh, gh, igf-1, igf-2, and igf-1r, and meanwhile elevated transcriptional levels of pi3k, akt1, tor, and s6k1 in muscle. The protein levels of p-AKT1^Ser473, p-TOR^Ser2448 as well as the ratios of p-AKT1^Ser473/AKT1 and p-TOR^Ser2448/TOR were clearly higher in LSS group. In comparison, hypothalamus-pituitary-interrenal (HPI) axis was more potently activated in the SSS group, as evidenced by the elevated levels of both brain corticotropin-releasing hormone (CRH) content and serum adrenocorticotropic hormone (ACTH) and cortisol concentrations. HPI axis-related genes were significantly upregulated in SSS group. These findings suggest that expanding stocking space mitigates the HPI axis activity and promotes fish growth via activation of the GH/IGFs axis and PI3K/AKT/TOR pathway.