Soft soil has low bearing capacity and poor engineering characteristics, so it can not be directly used as foundation for engineering construction. In addition, China’s industrial waste residue has a huge output and low utilization rate. In order to promote the requirements of high-quality development, this paper uses fly ash as precursor, carbide slag and sodium sulfate as activators to study the solidification mechanism and microstructure development of soft soil. The results show that the optimal mix ratio is 17.3% (mass fraction, the same below) of fly ash, 7.3% of carbide slag and 5.0% of sodium sulfate. The 7 and 28 d unconfined compressive strength of solidified soil under the optimal ratio are 2 474.0 and 3 134.0 kPa, respectively, and the error is small compared with the predicted strength. The response surface method is effective and scientific to optimize the mix ratio. The water stability coefficients of solidified soil at 7 and 28 d are 0.73 and 0.85, respectively, indicating that the solidified soil has good water erosion resistance. The hydration products of 28 d solidified soil are mainly hydrated calcium silicate (C-S-H) and ettringite (AFt), which fill and wrap the internal pores of solidified soil. The microstructure of solidified soil is gradually dense and the mechanical properties are improved.