Background <p>Bright light therapy (BLT) has been proved to have beneficial effects on Parkinson’s disease (PD), the mechanisms remained unclear. Improvements of visual pathways might be key to BLT.</p> Objective <p>The aim of this study is to validate whether BLT improves clinical symptoms in PD and explore the possible mechanisms of visual pathways evaluated by optical coherence tomography (OCT), pattern electroretinogram (PERG) and visual evoked potentials (VEP).</p> Methods <p>Twenty-three PD patients were enrolled in this crossover randomized placebo-controlled study. Participants received either one month of BLT or dim light therapy (DLT), separated by one-month wash-out period, followed by another intervention. Participants underwent clinical scales, and visual-related evaluations including OCT, PERG and VEP before and after each intervention. Mixed-effects regression models were used to determine the effect between BLT and DLT on improving the differentials of clinical scales (Δscales), OCT (Δretinal thickness), PERG (ΔPERG values) and VEP (ΔP100 latencies). Correlations between clinical symptoms and visual evaluations improvements were analyzed in PD patients receiving BLT.</p> Results <p>Excessive daytime sleepiness, anxiety, life quality and autonomic function were improved after BLT. Compared with DLT, bilateral ΔN95 latencies for PERG and ΔP100 latencies for VEP were improved after BLT. We did not observe the changes of four quadrants retinal nerve fiber layer (RNFL) thickness after BLT or DLT.</p> Conclusions <p>BLT is a valuable and safe non-pharmacological intervention for improving visual function in PD patients.</p> Significance <p>These findings extend neural mechanisms of BLT to visual pathways improvements.</p>