The workability of excessively wet subgrade can be effectively improved by using a “green materials” based entirely on industry byproduct of combination of fly ash (FA) with carbide lime (CL) and flue gas desulfurization gypsum (FGDG). This paper presents the results of a further systematic investigation on the mechanical behavior and durability of FA/CL/FGDG treated silt from Dezhou in Shandong province. FA, CL and FGDG were added at 3 levels in percentages of 8%-24%, 2%-6% and 1%-3% by dry weight of the soils, respectively. The unconfined compressive strength as well as splitting tensile strength (STS), volumetric expansion (VE), California bearing ratio (CBR), dry-wet cycles test and freeze-thaw cycles test were performed. The best strength of 0.802MPa was achieved for 24%FA/6%CL/1%FGDG treated soil, while the 16%FA/6%CL/1%FGDG treated soil also produce good strengths of 0.742MPa, after 7 days curing. STS keeps increasing with curing period in the first 8 weeks throughout the investigation. According to the VE test, a mellowing period may help prevent swelling from ettringite precipitation by letting these minerals form and hydrate before compaction. The FA/CL/FGDG treated soil has a better durability with strength loss ratio less than 15% after 10 wet-dry cycles, and the maximum weight loss around 25% after 10 freeze–thaw cycles. CBR decreases with decrease of compaction degree, but the values are still around 70 even in a poor compaction degree of 93%, meeting the general requirements of subgrade. So, if the silt subgrade is in excessively wet state and could not be compacted to the required density, the modified subgrade would still serve as a strong platform and could provide necessary support for pavement.