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As a low temperature and low cost synthesis method, the hydrothermal deposition is widely used to grow ZnO nanowire arrays. Their optical properties are closely related to concentration of the structural defects in the ZnO nanowires. In this work we report on investigation on the evolution of the structural defects of ZnO nanowire arrays for different growth time, before and after post-annealing. The photo-emission properties of the ZnO nanowire arrays have been investigated in detail by using photoluminescence (PL) spectroscopy both on the as-grown nanowires and on the annealed ones. The PL analysis results showed an increasing intensity in the visible emission band with the growth time which is closely related to the change of the concentration of the structural defects in the ZnO nanowires regarding to the UV near-edge emission. The visible PL emission band can be deconvoluted into three Gaussians components, which correspond to the green, yellow-orange and red emissions, respectively. It is further revealed that the intensity of the orange and red emission bands increase with the increase of the growth time, which are ascribed to the interstitial oxygen and/or hydroxide groups and adsorbed oxygen, respectively. The correlation between energy-dispersive X-ray spectroscopy (EDS) analysis and PL measurements revealed that the excess zinc can be formed in the nanowires during the hydrothermal process.