This paper focus on low frequency oscillations induced by the flow characteristics deviation of steam turbine. The causes and features of flow characteristics deviation are analyzed and described first. The coupling model between thermodynamic system and power system was established for time-domain simulation analysis, in an attempt to find out the effect mechanism of flow characteristics deviation on low frequency oscillations. The effect of flow characteristics derivation on the responses characteristics of turbo-generator is analyzed through simulation. Results indicate that the flow characteristics deviation is apt to cause low frequency oscillations. Both the amplitude and the speed of oscillations are related to the extent of flow characteristics deviation.