Negative air ion (NAI) is an important index for measuring air quality and is affected by vegetation and the environment in the forest. However, forest ecosystems are an essential source of NAI, and we know little about how the interactions are between forest vegetation and the environment, and its effects on NAI. In this paper, we selected Quercus variabilis as research subjects in warm temperate forest ecosystems and obtained meteorology, soil properties, air cleanliness, and vegetation photosynthesis data based on long-term positioning of automatic observation equipment. Finally, we analyzed the influence mechanism and contribution potential of forest vegetation and environmental elements on NAI using Pearson correlation analysis and partial least squares structural equation modeling (PLS-SEM). The results showed that the contribution of the environment elements and vegetation photosynthesis to NAI were significantly different, the contribution potential of vegetation photosynthesis was 62.65%, and the environment elements were 37.35%. The contribution of solar radiation and vapor pressure deficit (VPD) was the highest among the environment elements, with 68.94% and 16.55% contribution rates. Vegetation photosynthesis and PM_2.5 mainly affected NAI through direct effects, while photosynthetically active radiation, ultraviolet radiation, soil temperature, humidity, and VPD mainly affected NAI through indirect effects. Therefore, the structural equation model can be used to clarify the impact of vegetation and environment on NAI in the forest. This study can improve a comprehensively understanding of the influence mechanism of the environment elements and vegetation photosynthesis on NAI in forest ecosystems, and provide a theoretical basis for assessing the contribution potential of forest ecosystems to NAI.