Photoluminescence (PL) spectra of ZnO nanowires exited with 355nm laser pulse at room temperature were measured. The narrow violet band (peaking at 382 nm) and a broad green band (peaking at 507 nm) were observed in the PL spectrum. The intensity of luminescence depends on the excitation power density and the saturation effect of luminescence was also observed. But the variation and saturation value are different for these two luminescence peaks. It is shown in experiment that the violet peak of ZnO nanowires appear red-shift as excitation power density increases. The mechanism,which includes luminescence peaks appearance,saturation value,luminescence intensity variation,wavelength red-shift,have been analyzed.