The effects of minerals compositions and chemical characteristics on expansive soil's swelling and shrinkage are analysed by engineering practice in this paper. It is proposed that the content of montmorillonite and the exchange capacity of cation (CEC) is main internal factors which influence swelling and shrinkage of expansive soil, and the increase of the content of montmorillonite will cause linear increase of the free swell of expansive soil's and the mechanism of influence arc analysed from microstructures of minerals.