Abstract: The influence of Li composition on VIS-light photorefraction, UV-light-induced absorption and UV-light photorefraction in pure LiNbO_3 crystals at low light intensity was investigated. The experimental results indicate that different defects dominate the photorefractive centers of pure LiNbO_3 with varied compositions. For VIS-light photorefraction, Q polarons are introduced to explain the weak photovoltaic effect in LiNbO_3 with low composition, and bipolarons are considered to be responsible for the enhanced photovoltaic field in LiNbO_3 with high composition. It is also found that the UV-light-induced absorption and UV-light photorefraction have similar composition dependence. They increase slowly with the composition below 49.70 mol; but boosts up very rapidly above this critical composition. These findings are explained by the two different stages in the evolution of photorefractive centers during the increase of the crystal composition, which also proves our suggestion that different defects dominate the photorefractive centers of pure LiNbO_3 with varied compositions.