A gas dissolving system using double mixers to generate a higher gas concentration in liquid is disclosed. The gas dissolving system includes two gas mixers, a degassing device, pressure valves, a pressure sensor, and a pump. Liquid flows through mixers to entrain gas therein, and it thus contains dissolved gas and undissolved gas. The liquid subsequently flows into the degassing device so that undissolved gas is released to the outside environment, and dissolved gas remains in the liquid. The liquid with dissolved gas combines with raw liquid and is diluted so that it becomes liquid with a desired gas concentration as the output. A high gas concentration of liquid is obtained after more cycles of fluid flow through the mixers to dissolve gas without combining raw liquid.