The invention relates to an improvement method of an expansive soil roadbed filling. Expansive soil has the characteristics that the expansive soil expands, becomes soft and loses the strength after meeting water; and the expansive soil shrinks and cracks after losing water, so that the engineering properties are poor. Quicklime and the expansive soil are piled in layers or in a plurality of layers; during the layered piling, watering is performed so that the quicklime is sufficiently digested; the watering quantity is determined according to the natural water content of mixed soil; after the still standing for 24 hours, through the reaction for one day and night, soil particles with the particle diameter not greater than 5.0cm in the mixed material of the quicklime and the expensive soil reach more than 80 percent of the total weight; the filling building requirements are met; the screening is not needed; furnace slag is mixed, and stirring is performed until the color of the mixed material is consistent; and layered milling is performed. Calcium and magnesium in the quicklime and clay mineral silicon and aluminum and the like in the expansive soil are used for interaction, so that the engineering properties of the expansive soil are improved; a certain amount of coarse aggregate furnace slag is mixed so as to form a framework effect; and the dry cracking-resistant performance of the mixed material is improved, so that the goal of improving the expansive soil roadbed filling is achieved.