Abstract: Using geocell-reinforced cushion to improve the stability of embankment over soft soils has been recognized, but researchers has not had consensus for the failure mode and stability evaluation method. The reinforcement effect and failure mode of embankment over soft soils are compared through three sets of physical simulation tests using non-reinforced, geogrid-reinforced and geocell-reinforced cushion. The results show that the failure modes of non-reinforced and geogrid-reinforced cushion embankment are circular slip failure. However, as for the geocell-reinforced cushion embankment, the sliding surface is in the form of an arc passing through the center of embankment bottom and all formed in the soft soils, not passing through the reinforced cushion and embankment fill, and its location is deeper than that of the other two groups. Based on the test results, the mechanism of improving the stability of embankment by using geocell-reinforced cushion is summarized. In addition, on the basis of the identification of failure mode, the evaluation method of stability and the limit filling height of geocell-reinforced cushion embankment over soft soils are deduced.