Abstract: Water is an important factor affecting the stability of rock mass engineering. In order to comprehensively an- alyze the influence of water on the instantaneous and creep mechanical properties of rock,two kinds of tests were de- signed:uniaxial compression strength test and creep test. The water absorption characteristics of red sandstone were an- alyzed by water absorption experiments. Based on this,the standard samples with different water contents were prepared. First,the uniaxial compression strength tests were carried out on nine groups of specimens with different water contents. The volume strain method and LSR (Lateral Strain Response) method were used to determine the four char- acteristic stresses of red sandstone under uniaxial compression: closure stress,crack initiation stress,damage stress and failure stress,thus,the quantitative relationship between water content and mechanical parameters of red sandstone was obtained. Results show that the characteristic stress of each stage on the stress-strain curve decreases in negative expo- nential form with the increase of water content. When the specimen reaches saturation,the characteristic stress tends to be stable. Due to the characteristics of high porosity,the characteristic stress of red sandstone decrease significantly in a short time. The difference between crack initiation stress σi and closure stress σc decreases with the increase of water content. This indicates that the increase of water leads to the enhancement of the nonlinear characteristics of red sand- stone. The loss coefficient of strength and elastic modulus increases exponentially with water content,and the total loss coefficient of strength and elastic modulus of saturated red sandstone are 0． 484 and 0． 334,respectively. In addition, five groups of specimens with different initial water contents were continuously immersed in an environmental chamber filled with water to conduct creep tests under the combined action of load and water,so as to obtain the influence of long-term water environment on the creep mechanical characteristics of red sandstone. Results show that the instanta- neous strain and steady-state strain rate of red sandstone increase exponentially with the increase of water content, whereas the time-dependent strain and time to failure decrease with the increase of water content. When the water con- tent tends to be stable,the creep behavior of red sandstone still changes significantly. The reason is that water in the environment is further migrated to the crack tip along the new cracks generated by creep,and the initial crack expands further. The experimental results emphasize not only the great water weakening effect on the instantaneous mechanical properties of high porosity rocks in a short period of time,but also the sustained effect of water on the creep mechanics properties of saturated rock under the combined action of load and water. In addition,the experimental data and fitted equation will provide a reference for the long-term stability assessment of rock mass engineering.