Light quantity and quality strongly influence plant growth. Solar radiation is one of the main environmental factors driving vegetation carbon assimilation. As one of the natural climatic variables, clouds change the fraction of diffuse radiation arriving on the land surface and can influence carbon dioxide exchange between vegetation and the atmosphere. In this study, CO₂ flux was continuously measured during the vegetative growing season (June-August) from 2006 to 2008 by the eddy covariance systems in a mixed plantation in the Xiaolangdi area, Henan province. The clearness index (k_t) was used to describe the effects of cloudiness on solar radiation. Clear mornings and afternoons were identified based on the change of k_t with solar elevation (β) at a half-day scale. The impacts of cloudiness on the net ecosystem exchange of carbon dioxide (NEE) were investigated to reveal the mechanism of changes in coordinate of solar radiation and other environmental factors on the net carbon uptake by the mixed plantation. The results showed that net CO₂ uptake was higher under cloudy skies than that under clear skies. Under cloudy sky conditions, light-saturated maximum photosynthetic rate (P_max) increased by 38%, 58% and 55% and ecosystem apparent quantum yield (α) increased by factors of 2.6, 1.9 and 2.2 for the mid-growing seasons in 2006, 2007 and 2008, respectively, when compared to clear sky conditions. This work indicates that net carbon exchange could improve in the mixed plantation under cloudy skies relative to clear skies. For a given solar elevation angle interval, when the sky conditions changed from clear to cloudy, total solar radiation received by the ecosystem decreased, and balance of diffuse and direct components of solar radiation received by the ecosystem changed as well. Correspondingly, other environmental factors (Ta, VPD, etc.) also changed. Those changes can influence carbon exchange between forest ecosystem and the atmosphere. During cloudy days, the increase in diffuse radiation received by the ecosystem was more easily absorbed by shaded leaves for photosynthesis in forest canopy with higher leaf area index. Photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) were enhanced with the increase of k_t, but diffuse PAR first increased then decreased with the increase of k_t. Under cloudy sky conditions, the light distribution is more even and the diffuse radiation received by the forest ecosystem is greater, which benefits photosynthesis of the shaded leaves. VPD is an important factor that affects stomatal conductance. Therefore, the decrease in VPD associated with cloudy conditions can enhance canopy photosynthesis. Temperature is one of the main environmental factors controlling ecosystem respiration. Air temperature declined with the reduction of k_t.Net CO₂ uptake reached its maximum when k_t ranged between 0.4 and 0.5 (the average value was 0.44). When the value of k_t was more than 0.5, net CO₂ uptake decreased. It is suggested that the increased diffuse PAR, decreased VPD and decreased air temperature under cloudy skies may improve ecosystem photosynthesis and reduce ecosystem respiration, leading to an increase in net carbon uptake in the mixed plantation ecosystem.