Research conclusions: (1) The calculating result of determination of the critical height of a embankment is more authentic on consideration of both the internal friction angle and the cohesion at the same time.(2)The calculating analysis shows the critical height should not be designed in the way of reduction of 2-3 m of the filling critical height and it should not be designed only on consideration of the filling critical height, but it should be designed on full consideration of the filling height, slope ratio, the train load and the ground parameters. Otherwise a conservative design will be made. (3)In the previous methods，the soil internal friction angle and cohesion are seldom considered, therefore, they have certain limitations. (4)The calculation formula presented in this paper on consideration of the soil internal friction angle and cohesion at the same time can be used for the preliminarily determination of the critical height of the embankment on the soft soil.