Dissolved oxygen (DO) is the basis of fish survival, and proper DO level is an important condition to ensure the normal growth of fish. Hypoxic environment is prone to disturb the normal breathing and metabolism of fish, which in turn affects their growth and survival. Gill tissue is the respiratory organs of fish and is in direct contact with the external environment. However, there are few reports on the molecular regulatory mechanism of genetically improved farmed tilapia (GIFT, Oreochromis niloticus) gill tissues in response to hypoxia. Here, we first examine the hypoxia-induced damage of gill tissue by hematoxylin-eosin staining, and then constructed miRNA and mRNA libraries of GIFT gill tissue at 96h of hypoxia stress by a high-throughput sequencing technology, each library has three biological replicates. Gill lamellae of GIFT showed capillary rupture and red blood cell enlargement and overflow under hypoxia stress. Transcription sequencing results showed that the clean reads of miRNA libraries were 9,627,953-13,544,660 the clean reads of mRNA libraries were 43,817,776-53,130,102. Based on the miRNA-mRNA pairs screening principles and mRNA sequencing results, we selected and verified seven differentially expressed miRNAs and their potential target genes. The sequencing results were consistent with the qRT-PCR validation results. These selected miRNA-mRNA pairs are mainly concentrated in the signaling pathways of immune response and metabolic regulation. This study provides new insights into the mechanisms of fish adaptation under hypoxic stress. Overall design: GIFT used in the experiment was divided into two groups: the acute experimental group and the control group, each group has four replicates. The design of hypoxia stress were referenced by Yuan et al. Each experimental group was placed in a 500L tank with a density of 30 tails/tank, and the dissolved oxygen conditions were the same as the acclimation condition. Based on the real-time readings of the dissolved oxygen meter in the experiment, the dissolved oxygen was controlled by adjusting the rate of nitrogen and air charged into the water. The acute experimental group reduced the dissolved oxygen to (1. 02 ± 0. 10) mg · L- within 30 minutes 1. The culture conditions of the control group were the same as acclimation condition, and they were fed normally during the experiment. The acute experimental group and the control group were sampled at 96 h after hypoxia stress, and 4 fish were randomly selected from each tank. When sampling, the experimental fish was deeply anesthetized with MS-222, and the second right gill arch were dissected out on the sterilization operation platform, divided into two, and quickly placed in a 1.5 mL centrifuge tube in liquid nitrogen. Quick-freeze used by liquid nitrogen, and then transfer to -80 ? refrigerator for storage. In addition, two fish from each tank were randomly collected to sample the second right gill arch, rinsed with physiological saline, and fixed in 4% paraformaldehyde for the preparation of paraffin section.