Abstract: Through the dynamic triaxial tests on water-rich sandy mudstone, the dynamic strain analysis is made on the water-rich sandy mudstone under cyclic loading, and the influences of dynamic stress, static deviator stress and vibration frequency are analyzed emphatically. The results show that there are two types of cumulative irreversible deformation of water-rich sandy mudstone under cyclic loading, i.e. stable type and destructive type. The development of deformation can be divided into the initial and the stable stages for the stable development curve, and the cumulative deformation during initial stage is of the total deformation of 40% to 50%, but the proportion decreases with the increase of stress level. For the destructive development curve, there are three stages: the initial deformation stage, the constant deformation stage and the accelerative deformation stage. The cumulative plastic deformation of the sandy mudstone grows with the increase of the dynamic stress and static deviator stress and reduces with the increase of the vibration frequency, especially at a higher dynamic stress, higher static deviator stress or less frequency. There is a critical dynamic stress under cyclic loading, which depends on rock property, stress level and vibration frequency.