Alien plant invasions in forests can severely threaten native biodiversity and ecosystem functioning. Canopy closure in deciduous forests can cause variability in light levels, but light levels in forest edges and evergreen forests are constantly high and low, respectively. We tested how light conditions and light variability affect the invasiveness of alien plants by growing five pairs of invasive and noninvasive alien species under conditions of high, variable (with high light in the initial phase and low light in the latter phase of the experiment), and low light intensities. The total duration of the experiment was 200 days. The alien plants were grown singly or competed with a native deciduous or evergreen tree seedling. Overall, invasive plants produced marginally more biomass than noninvasive plants. From high to low (averaged across variable and low) light conditions, biomass of invasive plants decreased less than that of noninvasive species (-26.7% versus -37.7%), indicating that invaders may have a greater capacity to invade forests than noninvasive plants. From low to variable light conditions, biomass of invasive plants increased more than that of noninvasive species (63.7% versus 48.9%), indicating that the advantage of invaders over noninvasive species is most pronounced under variable light conditions. Competition with the deciduous tree reduced biomass more for the noninvasive plants than for the invasive plants, indicating that interspecific competition may further explain why some alien species can invade while others cannot. Our results indicate that high light early in the season benefited more for the invaders, and reduced irradiance from the middle of the experiment limited growth of the noninvasive plants more than that of the invasive plants. Together, our results suggest that variable light levels, simulating those in deciduous forests or caused by disturbances, may promote the invasiveness of alien plants.