Elevated atmospheric CO2 has direct photosynthesis effects on photosynthesis, respiration and water use efficiency, as well as on the growth and reproduction of plants. CO2 enrichment is more beneficial to plants with the C3 photosynthetic pathway than to those with the C4 photosynthetic pathway. Most major weeds are C4 plants, while many crops are C3 plant. Therefore, different effects of elevated atmospheric CO2 have important implications for weed/crop interaction. In this paper, the effect of CO2 enrichment and global chang on weed/crop interaction were summarized, and some theoretical and practical aspects were discussed.