Underwater hyperbaric dry welding method is one of the key technology for emergency repair of underwater pipeline leakage. Since the ambient pressure grows with water depth for application of the underwater dry hyperbaric welding method, the normal GMAW welding process tends to be unstable with the increase of the ambient pressure, which leads to the decline in the quality of welding. The cold metal transfer (CMT) welding method adopts a push-pull wire feeding mode and it has adaptive ability to control droplet transfer. In order to improve the welding quality under the hyperbaric environment, the experiments using the CMT welding method were conducted in atmospheric pressure (0.1 MPa) and 0.5 MPa environmental pressures respectively with a test system simulating the underwater hyperbaric environment. API X65 pipes were used as the base metal for welding experiments. A high-speed video camera was used to monitor the behavior of the welding arc. The welding processes at both ambient pressures were found to be stable. However, compared with the atmospheric environment, the CMT welding arc contracted at the ambient pressure of 0.5 MPa, and the droplet transfer frequency was reduced a little. Mechanical performance tests and microstructure analysis of the welds were carried out after welding. While welding in the hyperbaric environment, the upper bainite structure emerged in the microstructure of the seam and the heat-affected zone (HAZ) because of the enhanced environmental cooling effect. The tensile properties of the welds were not changed significantly. Although the low temperature impact toughness decreased, the test data were higher than the relevant limitations of standard. The experimental results show that the stability of the welding process is improved by applying the CMT welding method in the hyperbaric environment. It was verified that the CMT welding method can meet the requirements of underwater hyperbaric welding.