Chitosan samples irradiated with different doses were modified by acylation, and then their water solubility and viscosity were determined. In addition, the structures of acylated products were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric/differential thermal analysis (TG/DTA), powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that the yield of chitosan acylation products decreased with increasing irradiation dose. Compared with chitosan, the water solubility and weight-average molecular mass of non-irradiated chitosan acylation products increased significantly and the corresponding viscosity declined dramatically. However, with increasing irradiation dose, the water solubility and weight-average molecular mass of irradiated chitosan acylation products showed no obvious changes, but the corresponding viscosity reduced significantly. FT-IR spectra and TG/DTA curves indicated that the hydrophilic group (carboxyl) was successfully introduced into chitosan via acylation. Meanwhile, XRD patterns and SEM images showed that the crystal structure of modified chitosan was damaged and porous structures were observed, which promotes the entrance of water molecules. Structure and properties are closely related; therefore, structural changes inevitably lead to changes in properties. Comparing the properties and structures of chitosan and its acylation products can provide a certain theoretical reference value for further applications of chitosan.