To elucidate the mechanism by which intercropping proso millet (Panicum miliaceum L.) with mung bean (Vigna radiata L.) increases proso millet yield and to determine how this higher yield results from maximization of resources use efficiency, we designed and conducted four strip intercropping row arrangements, including two rows of proso millet altating with two rows of mung bean (2P2M), four rows of proso millet altating with two rows of mung bean (4P2M), four rows of proso millet altating with four rows of mung bean (4P4M), two rows of proso millet altating with four rows of mung bean (2P4M), sole proso millet (SP, control) and sole mung bean (SM, control) in Yulin, Shaanxi, China. Photosynthetically active radiation (PAR) in the canopy, radiation use efficiency (RUE), leaf photosynthetic characteristics, dry matter accumulation and allocation, and yield of proso millet were investigated. The results showed that the intercropping systems had higher PAR than the monoculture. Mean PAR intensities were increased by respectively 2.2％–23.4％, 19.8％–59.7％, and 61.2％–133.3％in the proso millet upper, middle and lower canopies compared with SP. The increase in PAR directly increased RUE, a result attributed mainly to the increase in photosynthetic capacity, including net photosynthetic rate and chlorophyll content. These responses resulted in increased dry matter allocation to plant organs. Yield of intercropped proso millet was 6.8％–37.3％higher than that under monoculture and the land equivalent ratios for the different intercropping patts were all greater than unity (>1). In general, yield followed a positive linear function of PAR in the intercropping system. The results indicated that intercropping can boost proso millet yield, evidently by altering light distribution within its canopy and consequently increasing RUE, thereby increasing leaf photosynthetic capacity, dry matter accumulation, and allocation to the grain. The optimum combination for improving the growth and yield of proso millet on the Loess Plateau of China was 2P4M.