Increasing water content in fixed beds is a prospective way to improve the gas storage density by means of hydrate formation. However, the high water content strongly hinders the hydrate formation. In this study, diesel oil was mixed into highly water-saturated activated carbon beds to establish gas channels so that to improve methane hydrate formation. Hydrate formation experiments in four activated carbons with different particle sizes were conducted at 274.15 K and 6.0 MPa. For wet carbon beds without diesel oil, both of the storage capacity of hydrate (S-w) and the storage density of bed (S-b) increased with an decrease in particle size. When the mass ratio of water to activated carbon was set to 2.0, the highest S-w of 67.98 V/V-w and corresponding S-b of 51.92 V/V-bed among the beds were obtained in the bed with particle size of 100-400 mesh. Sodium dodecyl sulfate (SDS) was found to suppress hydrate formation in fine-particle beds. By adding diesel oil, both the formation kinetics and S-w in highly water-saturated beds especially in small-particle ones were significantly improved. The S-w and S-b in the bed with particle size of 100-400 mesh were increased to 180.41 V/V-w and 111.75 V/V-bed, respectively. Furthermore, by adjusting the proportions of water, oil and activated carbon in the bed, the S-w was further improved. The successive gas storage tests suggested that the oil-contained bed can be simply renewed by manual stirring. Gas recovery experiments showed that the hydrate dissociated faster in the oil-contained bed than that in SDS solution, suggesting more efforts should be made to control the hydrate dissociation to drive this method into practical application.