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Identifying the anthropogenic influence on the spatial distribution of plant diversity can provide reference for ecological conservation. In this study, an identification method for the anthropogenic influence was proposed by observing multiple gradients in estuarine areas. The multiple gradients included soil, vegetation, heat and aridity, distance, and landscape gradients; the former three are direct gradients and the latter two are indirect ones. The single and comprehensive effects of the multiple gradients on plant diversity were analyzed. Anthropogenic influence was then identified by combining the influence degrees of human activity through different gradients and the effect weights of different gradient factors on the plant diversity. A new index, namely, anthropogenic influence index (AII) was established to quantify and spatially express the influence. Chongming Island, the largest estuarine alluvial island in the world, was selected to demonstrate the method. Results validated the effectiveness of our method. The multiple gradients determined the spatial distribution of plant diversity. The direct gradient factors exhibited significant gross and net effects, whereas the indirect gradient factors showed significant gross but insignificant net effects. Of all the gradient factors, the normalized difference vegetation index contributed the most to the spatial distribution of plant diversity. The AII was 53.38% over the entire study area, which indicated that anthropogenic influence has been the main driving factor of the spatial variation of plant diversity. The differences of AII across different plant communities and nature reserves revealed that human activity generally increased the plant diversity on Chongming Island. Keywords: Anthropogenic influence, Multiple gradients, Plant diversity, Spatial distribution, Estuary, Chongming island