Abstract: The expansive soil is one of the recognized challenges in the field of geotechnical engineering. Based on the microbial technology, combined with the improved materials such as lime, cement and fly ash, the experimental researches on the engineering characteristics of composite improved expansive soil are conducted. The results show that: (1) The weak expansive soil is successfully changed into the non-expansive one by the microbial composite improvement of expansive soil, and the free expansion rate is reduced to 8% at the lowest, with a decrease of 84.31%. The engineering effect is good. (2) The decrease of the relative content of montmorillonite and the increase of the relative content of illite in the composite treated samples are the important reason for inhibiting the expansion and contraction characteristics. (3) When the bacterial solution of 20% is added to the expansive soil, the friction angle can increase by 23% compared to that at the same time last year. (4) In the case of composite improvement of expansive soil with bacterial solution, fly ash and lime, a low ratio combination of lime of 1% and fly ash of 2% is more advantageous for improving the strength.