The water content of weak expansive soil used in subgrade constructed with wrapping method usually varies in a certain interval. Current researches are focused on expansive amount under saturated status rather than that under certain intervals of water content. To investigate the expansive deformation of weak expansive soil with different moistening degrees,one-dimensional expansion test was conducted on “equivalent” remolded expansive soils moistened to different water contents. The “equivalent” here means the same soil type with identical initial moisture content and dry density. Results showed that when water content reached 25%, further increasing the water content has little impact on the expansive deformation. On this basis,the concept of converted expansion (converted linear expansive ratio and converted swelling force) is put forward, which is defined as the difference between the linear expansion of original specimen moistened until saturation and the linear expansion of original specimen moistened until any random water content.Through analyzing the construction water content and equilibrium water content inside the subgrade, the interval of water content inside subgrade which remarkably affects expansive deformation amount was obtained. It is concluded that the construction water content shall be controlled strictly in embankment filling.