The characteristics of the exciton in ZnO is essential to the development of ZnO-based optoelectronic devices, therefore it is critical to study the optical properties of exciton and transition process in ZnO quantum dot. In the present work, zinc oxide nanocrystals were prepared by the sol-gel method. X-ray diffraction (XRD) pattern shows that they have a hexagonal wurtzite structure. The pumping power dependent UV emission and their dynamics were studied, the spontaneous emission of the free exciton and the stimulated emission due to exciton-exciton collision and electron hole plasmon were observed, and the pumping-power dependent dynamics was reported for the first time. This is helpful for us to understand the near-band-edge excitonic emission, and might be valuable in the realization of semiconductor UV diode.