Aiming at a series of problems such as large amount of fly ash (FA), low utilization rate and serious desertification of local soil in Ximeng Region of Inner Mongolia, this work put forward a new idea of using fly ash to improve desert soil. Firstly, the inert fly ash was activated by sulfuric acid. Scanning electron microscope (SEM) and X-rays diffraction (XRD) were used to study the structural changes of fly ash. X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TG) were used to analyze the surface hydroxyl of fly ash qualitatively and quantitatively, so as to clarify the surface hydroxylation effect of fly ash. Secondly, the influence of modified fly ash on the stability of sand soil was studied by using fly ash before and after modification as soil reinforcement agent. The results showed that the number of hydroxyl groups on the surface of the particles increased by 4 times compared with the original fly ash after pre-modification with 1.5 mol/L sulfuric acid solution. According to the mass ratio of 1:9, fly ash before and after acid modification was mixed with sand and stood for 15 days, respectively, and the mechanical strength of the mixed soil was measured. The cohesion between the original sand was 0.29 kPa, and the cohesion between fly ash and sand before modification was 0.88 kPa. However, the cohesion of the modified fly ash-sand mixture was increased to 3.51 kPa. The fly ash-sand composite soil prepared by adding modified fly ash and sand soil had the functions of sand fixation, water retention, nutrient supply and soil aggregate structure reconstruction.