Carbon (C) and nitrogen (N) contents, along with the corresponding C:N ratios in crops, significantly impact C and N cycles in cropland ecosystems. However, the effects of climate change and the increasing frequency of extreme weather events (EWEs) on the C:N ratios of major crops remain uncertain. Here, we combined field experiments and mathematical modeling to explore the impact of excessive precipitation, precipitation shortage, and heat events on the C and N contents and C:N ratios of rice and wheat in a major grain production base in China. The results showed that the average C:N ratios for different rice organs during the growing season ranged from 26.8 to 47.6, while winter wheat C:N ratios ranged from 26.4 to 41.1. The calibration model based on measured C:N ratios effectively improved the simulation of crop C and N contents. Excessive precipitation led to a 25 %-37 % increase in wheat C contents and a 66 %-76 % increase in N contents, while plant C:N ratios decreased by 30 %-37 %. Precipitation shortage reduced C and N contents in various wheat organs by 4 %-29 % and 8 %-47 %, respectively, while increasing corresponding C:N ratios by 4 %-33 % due to larger reductions in N than C contents. Extreme heat decreased C content in rice grains by 12 %-13 % and N content by 8 %-24 %, while C content in rice leaves decreased by 27 %-30 % and N content by 34 %-41 %. However, C and N contents in roots increased by 13 %-18 % and 33 %-48 %, respectively, leading to an average increase in the C:N ratios of rice grains and leaves by 5 % and 14 %, while the C:N ratio of roots decreased by 26 %. In the future, the effects of heat events on grain C:N ratios are projected to be even more pronounced.