High sulfate content in cement carries risks of volume expansion in late hydration. Nano-TiO2 and nano-SiO2 were used to modify semi-dry flue gas desulfurization ash, which contains a high rate of CaSO3·0.5H2O, and nano-modified semi-dry flue gas desulfurization ash was used to prepare nano-solid waste high sulfur cement, to solve the problem of poor durability caused by high CaSO3·0.5H2O content in the matrix. The ratio of each component in nano-solid waste high sulfur cement was determined according to the stability, water requirement of normal consistency, setting time and compressive strength of nano-solid waste high sulfur cement. LPSA was used to analyze the particle size distribution of raw materials. The water contact angle measurement was used to analyze the wettability of hardened slurry, the XRD was used to analyze the mineral composition of raw material and hardened slurry, the FTIR was used to analyze the change of microstructure of raw material and hardened slurry, the SEM was used to analyze the micromorphology of raw material and hardened slurry. The results show that the particle size distribution range of semi-dry flue gas desulfurization ash is 0.31~127.38 μm, which is wider and finer than that of cement particles, and so can optimize the grading range of cement. The semi-dry flue gas desulfurization ash could delay the setting of cement hydration, prolong the setting time, and reduce the compressive strength, especially with a large amount. Adding nano SiO2 and nano TiO2 can reduce the water requirement of normal consistency of cement matrix and improve its compressive strength. The synergistic modification of 3% nano TiO2 and 2% nano SiO2 can effectively stabilize CaSO3·0.5H2O in semi-dry flue gas desulfurization ash, further stimulating the potential activity of semi-dry flue gas desulfurization ash and improve the mechanical properties of cement hardened slurry. The 28-day compressive strength of modified nano-solid waste high sulfur cement is 64.72 MPa, 83% higher than that of unmodified high sulfur cement and even 16% higher than that of pure cement. The wetting edge angle increases to hydrophobic change, which is conducive to improving durability. XRD analysis results show that the content of AFM-like mineralsin hydration products is shallow, which reduces the risk of expansion. FTIR analysis showed that the stretching vibration peak of —OH contained in Ca(OH)2 in the hydration system was enhanced, further improving the hardened slurry’s chemical erosion resistance. SEM analysis shows that the hydration product has uniform texture and fewer microstructure defects. Nano-TiO2 and nano-SiO2 co-modified semi-dry flue gas desulfurization ash can stabilize sulfate and sulfite and are used to prepare high-performance nano solid waste high sulfur cement is beneficial to carbon reduction, energy conservation and environmental protection.