Abstract: The degradation of the mechanical characteristics of red bed weak rock at saturated state directly affects the deformation stability of the related engineering structure. The characteristics of compressive strength and tensile strength,and deformation and failure characteristics and microstructure characteristics of red bed weak rock were tested and the test analysis systematically considered the drying, natural and saturated state. The results are as fol- lows:① Softening characteristics of red bed weak rocks at saturated state were significant and the ductility features of failure were more obvious. The softening coefficient of compressive strength of rock specimens under different confining pressures was 0. 36 - 0. 65. The elastic modulus and deformation modulus decreased gradually and was reduced by 35． 60% -75. 53% ,44. 57% -69. 64% respectively relative to the dry state. Also,the smaller the confining pressure was,the more obviously compressive strength and modulus decreased. ② The cohesive force and internal friction angle of rock specimen at saturated state fell 47. 77% and 12. 68% respectively relative to the dry state,and the cohesive force was more sensitive to the moisture state of rock specimen. ③ The softening coefficient of tensile strength of rock specimens at saturated state was 0. 40,and the characteristic of the local failure near load line was particularly obvi- ous. ④ Because of the dissolution damage of argillaceous cement among the mineral particles of the rock specimens and the swell and decomposition of clay minerals such as illite and montmorillonite,rock mass structure changed from dense state to porous and loose state gradually and the pores were filled by water. The microstructure change directly led to the degradation of the mechanical properties of red bed weak rock.