Localized vibration caused by the metro operation changes the mechanical properties of underground soft clay, which can cause engineering geological problems, such as continuous subsidence of metro tunnels, but the mechanism is still unclear. In order to study the cause of tunnel’s continuous settlement, this paper studies the flow characteristics of underground soft clay under different water contents, vibration frequencies and confining pressures by the pulling sphere experiment. The experimental results indicate that the flow characteristics of soft clay increase with the increasing water content and vibration frequency, and decrease with the increasing confining pressure. However, the effect of increasing water content on the fluidity of soft clay is more obvious at lower water content, and becomes no longer significant with the gradual increase of water content of soil sample. Confining pressure and vibration frequency have cross-effects on the flow characteristics of soft clay. Under high vibration frequency, the confining pressure has little effect on the flow characteristics of soft clay. At presence of high confining pressure, the increase of vibration frequency will affect the flow characteristics of soft clay only if the vibration frequency exceeds a critical value.