X-ray diffraction spectrum, Raman spectroscopy and UV-Vis transmission spectroscopy are important techniques for detecting thin films. By analysing these spectral properties, we can obtain the information of the phase, crystal structure and light transmission properties of the films. The films with different thicknesses were prepared on a quartz glass substrate by sol-gel and spin coating method. The X-ray diffraction, Raman spectroscopy and UV-Vis transmission spectrum detection of the samples were carried out. Firstly, the X-ray diffraction shows that the films exhibit peak (002) crystal plane. The ZnO thin films are hexagonal wurtzite structures and grow along the c-axis preferred orientation. With the increase of the film thickness, the (002) peak is enhanced, and the grain size of the ZnO grows with the increase of the film thickness. Scanning electron microscopy analysis of the film's surface morphology also shows that the film surface is compact and uniform, with nanocrystalline structure and obvious hexagonal shape. Secondly, the Raman spectrum detection shows that 437 cm^-1 Raman peak appears in all samples, which is the characteristic Raman peak of the ZnO wurtzite structure. With the increase of film thickness, the intensity of 437 cm^-1 peak also increases, which further indicates that the crystallization of film is strengthened with the increase of film thickness. Finally, It is found that the absorption edge of the film has a certain redshift with the increase of the film thickness based on that of the UV-Vis transmission spectrum. The transmittance of the film in the visible region decreases slightly with the increase of the film thickness, but the average transmittance is above 90%. On this basis, the further theoretical calculation of the experimental results shows that the change of thickness has little effect on the refractive index of the film samples, but the bandgap width becomes narrow with the increase of the thickness of the film. However, the bandgap width of undoped ZnO films is larger than the theoretical value of 3.37 eV. Further analysis shows that the change of ZnO film thickness is positively related to the change of ZnO grain size. Essentially, the change of absorption edge or optical band gap is caused by the change of ZnO grain size.