This study empirically examines the impact of financial investment on environmental sustainability in publicly traded companies at the Shanghai and Shenzhen stock exchanges in China from 2011 to 2021. Using a generalized additive model (GAM), we scrutinize this relationship, revealing a consistent linear trend. Additionally, a panel model uncovers a crowding‐out effect of financial investment, particularly pronounced in firms with weaker environmental performance. Corporate financial investment primarily undermines environmental sustainability by cutting down primary business investment, inhibiting green innovation, and diminishing human capital investment. Furthermore, in regions with favorable institutional environments, financial investment transitions its impact on corporate environmental sustainability from crowding out to reservoir effects. This shift is particularly pronounced when the financial regulatory index and regional marketization index surpass the respective thresholds of 0.314 and 0.618. This evidence indicates that it is pertinent to circumvent the adverse impacts of corporate financial investment on environmental sustainability at the institutional level.