In the rice-crayfish co-culture system, natural resources and primary productivity of rice fields are abundant, which provides abundant natural baits for Procambarus clarkii. At present, there is no report about the utilization of rice fields with P. clarkii. In this paper, C/N was increased by adding carbon source to feed, which makes nitrogen utilization increase and enriches the density of natural baits in rice fields. We designed three feeding modes as group A (formulated diet C/N: 8), group B (formulated diet and bran C/N: 12) and group C (formulated diet, bran and glucose C/N: 16). Stable isotope techniques were used for analyzing the feeding habits of P. clarkii. The results showed that: ①The δ¹³C and δ¹⁵N values of muscles in different dietary C/N groups were significantly lower than the juvenile P. clarkii muscle, and the δ¹³C values of adult P. clarkii muscle decreased with the increase of C/N. While the δ¹⁵N values of P. clarkii muscle in group B with different sizes were higher than the other two groups. In the same C/N treatment, the δ¹³C and δ¹⁵N value of crayfish muscle of a small size crayfish were the highest. ②According to the analysis of diet, formulated feed was the first food source of P. clarkii with different sizes in group A, accounting for 38.03%-44.17%. Formulated feed was still the first contribution of crayfish diet with different sizes in group B but dropped to 22.13%-25.35%. In group C, the organic debris for the diet of small size P. clarkii accounted for 19.93% was the first food source. Formulated feed which was the first food source of middle and large size P. clarkii declined further to 18.85% and 19.4% respectively. The natural food proportions including zooplankton, algae and Alternanthera philoxeroides were significantly increased in groups B and C compared with group A. The results indicate that by increasing the C/N of feed of rice-crayfish co-culture system, the utilization rate of natural food in the rice fields can be increased, and the excessive use and waste of the feed can be avoided, and it is beneficial to realize the rational utilization of rice fields resources.