<p>Diffuse light has potential to increase ecosystem gross primary productivity without the confounding effect of other environmental factors. However, the magnitude of the importance of diffuse light for ecosystem carbon uptake and the mechanism behind the diffuse light-related photosynthetic enhancement is unclear. Here, 2 years of gross ecosystem primary productivity (GEP), assessed by eddy covariance technology over a (winter) wheat cropland, was used to determine whether diffuse photosynthetic active radiance (PAR<sub>dif</sub>) affected wheat GEP. Additionally, the method of Artificial Neural Network combined with interference analysis and modelling were used to quantify the relative importance of diffuse light for GEP variations and to explore the underlying mechanism of diffuse light effect on GEP. Wheat GEP increased significantly with increase in PAR<sub>dif</sub> in the absence of effect of total PAR. PAR<sub>dif</sub> was found to be the most important factor for wheat GEP, making a contribution of 41.3 % in 2011 and 35.7 % in 2012 to GEP variations, which were greater than the contribution of total PAR, air temperature, vapor pressure deficit and friction velocity. The results of combination of model and measured data indicated that as PAR<sub>dif</sub> increasing, the within canopy, especially the middle and lower canopy, intercepted more light, leading to photosynthetic increase in entire canopy. Over all, our study provided a new evidence for the importance of diffuse light for carbon uptake in agroecosystem, which is importance for predicting the response of ecosystem carbon budget to future light climate changes.</p>