<p>Lakes across China are increasingly shaped by climate change and human activities, yet how these pressures have altered the underwater light environment remains poorly quantified. Here we combine in situ measurements, satellite observations and deep learning models to quantify long-term changes in underwater light environment across 1278 lakes in China from 1990 to 2020. Underwater ultraviolet radiation increased by 0.14 W m⁻² yr⁻¹ decade⁻¹, consistent with a weakening of light attenuation in the water column (−0.24 m⁻¹ decade⁻¹), meaning that light penetrates deeper. The dominant drivers differ by region: reduced particle inputs to plateau lakes are linked to increasing vegetation in surrounding environment, whereas reductions in plain lakes are most consistent with urbanization. Increased penetration of ultraviolet radiation may suppress aquatic growth and alter ecosystem structure, highlighting an emerging pathway by which climate and land-use change can reshape lake habitats.</p>