A large amount of fly ash is produced every year in China, and there are big differences in the disposal and utilization of fly ash with different particle size. In order to explore the differences in the composition and structure of fly ash with different particle size, Gujiao fly ash was selected as the research object, and sieved into 8 size fractions. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) was used to characterize the samples. XRD pattern shows that the dominant phase is amorphous glass (61.93%～74.76%), mullite (20.45%～29.59%) and a small amount of quartz (1.23%～5.64%). As the particle size increases, the mullite content decreases, the quartz content increases first and then decreases, and the glass exhibits an overall upward trend. FTIR shows that Si—O (Si, Al) asymmetric stretching vibration is main chemical bond (58.86%～67.39%), which is mainly provided by aluminosilicate in glass, followed by Si—O—(Si) bending vibration (15.28%～21.49%), Si—O—Si symmetric stretching vibration (6.18%～9.67%), and Si—O—(Al) symmetric stretching vibration (0.79%～4.02%). With the increase of particle size, the relative content of Si—O(Si, Al) asymmetric stretching vibration increases, Si—O—(Si) bending vibration decreases, while Si—O—Si symmetric stretching vibration and Si—O—(Al) symmetric stretching vibration does not show an obvious change trend. The relationship between XRD content and FTIR chemical bond content shows as Si—O (Si, Al) asymmetric stretching vibration is the main absorption peak of glass, and the content of this band increase with particle size is consistent with the increase of glass obtained by XRD. For quartz, the content got by XRD quantification in different size fractions are also same with the changing trend of the Si—O—(Si) bending vibration near 464 cm-1 and Si—O—Si asymmetric stretching vibration near 1 090 cm-1. The relative content of the strong absorption peak at 556 cm-1 of mullite (y) obtained by FTIR and the mullite content (x%) obtained by XRD shows a linear relationship as y=0.396x-1.997 with 0.868 for R2.