Mudstone with low content of clay minerals is determined not expansive by existing railway standards. However,it is still of weak expansion in water,and has potential damage to high-speed railway which is tolerable to a deformation of only 4 mm. In this article,we take the reshaped mudstone with low content of clay minerals in the foundation of a high speed railway as research object, and analyze the effects of initial water content, dry density and overburden load on the bulging limit condition by expansion test under overburden load. Results show that:1) expansion ratio of mudstone in bulging limit condition increases with the increase of dry density when initial water content and overburden load are stable;2)the greater the initial water content is, the smaller the expansion rate in bulging limit condition is when dry density and overburden load are stable;3) expansion rate and water content in bulging limit condition both decrease with the increasing of overburden load when the initial water content and dry density are stable. Furthermore, we obtained the fitting calculation formula of expansion rate in bulging limit condition of mudstone. Verification result shows that the difference between measured value and predicted value by the formula of expansion rate in bulging limit condition is small. The fitted formula could be used to predict expansion in practical engineering to provide guidance for engineering.