Abstract: Back pressure is extensively applied in triaxial tests to improve sample saturation and has been identified irrelevant to the strength of saturated soils. However, the influence of back pressure on macro-mechanical properties of methane hydrate soils has been shown by other researchers. This paper aims to further investigate the geotechnical paradox. Firstly, the mechanism of the back pressure effect on mechanical properties of methane hydrate soils is studied according to its microstructure. Secondly, DEM biaxial tests incorporating the micro contact model of methane hydrate soils are performed to examine the proposed mechanism. Finally, 20 DEM biaxial tests are simulated to discuss the specific influence of back pressure on macro-mechanical properties of methane hydrate soils. The results show that the influence of back pressure is related to the properties of methane hydrate. The back pressure improves the shear strength, increases the tendency of strain softening, and dilatancy and affects the elastic parameters of methane hydrate soils. In addition, when the back pressure is high, the influence on strength parameters can not be ignored while the influence on elastic parameters of methane hydrate soils is negligible.