Abstract: The old clay in the karst area can change its dynamic deformation characteristics and weak the intensity parameters correspondingly under the dynamic load caused by human engineering activities, which may leading to the structural damage on soil cave eventually. The old clay in Penghu Bay of Wuhan City was taken as the study object. Combined with the engineering geological conditions of the karst collapse site, the indoor dynamic triaxial test of the old clay was carried out to study the dynamic deformation characteristics and the strength weakening law under the influence of the factors such as dynamic stress ratio, confining pressure and cycle times, then the strength weakening model under cyclic loading was proposed. The results show that the pore water pressure of the old clay increases with the increase of the confining pressure and the dynamic stress ratio and the number of cycles. When the dynamic stress ratio and the cycling numbers do not exceed a certain critical value, the strength of the soil increases with the enhancing of dynamic stress ratio and the number of cycles, that is to say the increase in pore pressure triggers the attenuation of soil strength. In the early raising period of confining pressure, the strength of soil increases with the addition of confining pressure. When the confining pressure increases until the soil structural damaged, the shear strength of the old clay is greatly reduced. The strength reduction rate decreases correspondingly with the increase of the confining pressure, dynamic stress ratio and the number of cycles. The correlation of the old clay strength weakening model obtained with empirical formula is fairly high. The weakening mechanism of the strength of the old clay is further obtained.