Abstract: Abstract: More than 5 billion dollars economic loss is caused by expansive soil each year around the world. Soil chemical amelioration is better than the other methods, but better effect and higher safety cannot be achieved by a single ameliorant. In order to weaken serious damages to civil engineering facilities and agricultural ecological environment induced by expansive soils, composed ameliorants to improve the expansive soil were tested in laboratory tests in this paper. The low free swelling ratio (no more than 40%) and the high shear strength were required for the soil amendment. Expansive soil used for testing was collected from Handan in Hebei province in China. It was a strong expansive soil with 130% free swelling ratio (more than 90%) and contained large hydrophilic minerals. Based on the physical-chemical effect of ion exchange (to decrease thickness of hydration shell), polymer film-forming effect (to prevent hydrone from permeating clay particle) and so on, the combinations of different ratios of CaO (1%-8%), KCl(3%-10%), PVA(0.3%-1.0%), polypropylene fiber(0.2%, 0.4%, 0.6%, 0.8%, 1.0%) were used to treat expansive soils. Based on the construction safety and experiment error, among 54 groups of free expansion ratio tests (using for measuring free swelling ratio), 5% KCl+0.8% PVA and 4% CaO+0.8% PVA had better improvement effect, and they were chosen for testing shear strength. A total of 5 groups were designed in triaxial compression tests: no amendment, 0.6% polypropylene fiber, 5% KCl+0.8% PVA, 5% KCl+0.8% PVA+0.6% polypropylene fiber, 4% CaO+0.8% PVA. They were tested for studying improvement effect by measuring axial force, axial strain and shear strength. The results showed that the improvement effects of composite ameliorants were remarkable. The free swelling ratio was reduced to 27% from 130%, and the shear strength was increased 15% by using the mixed solution of 0.8% PVA+ 5% KCl+0.6% polypropylene fiber. The free swelling ratio was reduced to 33% from 130%, and shear strength was increased 110% by using the mixed solution of 0.8% PVA+4% CaO. In addition, the research showed that the reduction of expansion rate due to adding KCl or CaO was reinforced by the film-effect of PVA and the increase of shear strength due to adding polypropylene fiber was weekend by KCl and PVA. Crack initiation of expansive soil was effectively reduced by adding polypropylene fiber and shear strength was improved too. The effectiveness of composed improvement methods of strong expansive soil was validated by the laboratory tests. The promotion and inhibition mechanism needs further research.