Hypoxia is one of the main environmental stress factors affecting the growth, reproduction, and survival of fish in aquaculture. In addition to the antioxidant defense system, the mechanism of hypoxia tolerance in fish also includes many regulatory factors that play an important role in hypoxia signal transduction. The calcium-sensing receptor (CaSR) is a transmembrane receptor belonging to the G-protein-coupled receptor (GPCRs) superfamily, and is widely conserved in mammals, fish, and other vertebrates. CaSR can participate in a variety of important physiological processes by regulating different signal pathways under the stimulation of Ca²⁺ and other agonists. Apoptosis is an important physiological activity in most organisms, and under normal conditions it promotes the development of the organism and helps maintain homeostasis. Oxidative stress induced by hypoxia can induce cell apoptosis. It was found that hypoxia upregulated the expression of CaSR, and increased CaSR promoted intracellular calcium overload, which induced endoplasmic reticulum stress, and could affect cell apoptosis genes in specific pathways, leading to downstream apoptosis. However, the regulation of CaSR on cell apoptosis under anoxic conditions is primarily observed in mammals and has rarely been reported in fish. The aim of this study was to investigate the characteristics of the CaSR gene and its regulatory mechanism involved in apoptosis during hypoxia stress in Genetically Improved Farmed Tilapia (GIFT). The cDNA full-length sequence of the CaSR gene of GIFT was cloned by RT-PCR. Quantitative real-time PCR (qRT-PCR) was used to analyze the expression pattern of CaSR in different tissues, and further detected expression changes in CaSR and the mRNA of other apoptosis-related genes in the liver under hypoxic conditions (0.55 mg/L). Finally, ELISA was used to detect changes in activities of antioxidant enzymes in the liver, while morphological changes and apoptosis in the liver were observed by HE and TUNEL staining, respectively. The results showed that the cDNA full-length sequence of the CaSR gene in GIFT was 3265 bp in length, including 21 bp at the 5′-UTR, 421 bp at the 3′-UTR, and a 2823 bp open reading frame (ORF) encoding 940 amino acids. The predicted protein structure contained a seven-transmembrane domain unique to the GPCRs superfamily. Multiple sequence comparisons of CaSR proteins indicated that CaSR has the highest similarity with the corresponding protein in tilapia. CaSR gene mRNA was found to be expressed in nine tissues, with the highest expression in muscle, followed by kidney. Hypoxic stress could lead to structural damage of liver and promote apoptosis of liver cells. Compared with the control group (5.0 mg/L), hypoxic stress could enhance the activity of SOD, CAT and GSH-Px antioxidant enzymes and maintain them at high levels, and could also significantly upregulate the expression of CaSR mRNA and cause mRNA expression changes in Bcl-2, Caspase-3, and P53, all of which are apoptosis genes. The results suggest that hypoxic stress could induce oxidative stress and cause damage in the tissues of GIFT, and that activated CaSR might participate in the process of hypoxia signal transduction through mediating Ca²⁺ to affect the expression of key genes in the apoptosis pathway under oxidative stress, thus leading to the apoptosis of fish hepatocytes.