ZnO thin films are deposited on SiO2 substrates by radio-frequency magnetron sputtering. The A and B excitons are observed from the absorption spectra at room temperature. The A, B and C excitons are observed from the reflectance spectra at 19K. These results indicate that the obtained ZnO films have a good wurtzite structure. We obtain the stimulated emission resulting from electron-hole plasma. The laser spectrum is observed, which consists of a large number of narrow peaks with an around 0.5nm spacing in between, corresponding to a self-formed cavity length of around 31.5μm. The self-formation of laser cavity in the ZnO thin film is closely related to its hexagon structure.