Corn，as a kind of C4 crop , was ever considered as the most important natural refuge for Bt susceptible cotton bollworm, Helicoverpa armigera, individuals in North China where Bt cotton was widely planted. However, direct and rigorous data that supported the Bt refuge function of non-Bt cotton cultivars in the small farm and mixed planting dominated area in North China under current planting pattern is still lacking. In this study, poplar traps were set up with renewing the poplar traps weekly to collect cotton bollworm moths in Bt cotton fields from August to September of 2006-2007. The captured moths were preserved in 95% alcohol for further use in later examination in laboratory. And the carbon stable isotope δ13C were determined to identify larval host types (C3 or C4) for the captured moths samples in North China, individually. 10 moths daily were selected among the captured days and totally 30 moths in 2006 and 20 moths in 2007 were selected for δ13C detection. Our results showed that 3, 4, or 5 among 10 moths from 3 samples selected as final samples of 2006, 4 and 9 out of 10 moths in 2007, were determined as C4 resource by the signature of δ13C in moth wings, indicating its larva fed on C4 photosynthetic plants such as corn cultivars. The left 7, 6, or 5 among 10 moths in 2006; 6 and 1 out of 10 moths in 2007 were divided into C3 resource group with the same criteria, indicating its larva come from C3 photosynthetic plants, such as Bt cotton, soybean or peanut. Considering the daily moth sample size captured by poplar traps during the whole third generation of cotton bollworm, 40.5%—56.8% cotton bollworm moths during the third generation in Bt cotton field came from C4 plants (such as corn cultivars), which was comparative with C3 resource (such as Bt cotton cultivars) individuals in population size. However, the emergence time of cotton bollworm moths from C4 phenotype individual was latter than that from C3 resource as a whole during 2007. In other words, more moths of cotton bollworm from C4 crops emerged in the latter than from C3 crops. Moreover, similar variation of moth resource was also observed in 2006. Our results demonstrated that C4 plants did serve as most important Bt refuge for the third generation cotton bollworm in North China, however exit a risk of becoming disabled for un-synchronization with C3 resource; therefore, suggesting that a lot of C4 phenotype moths did enter Bt cotton fields for mating and eggs laying which supplied Bt susceptible individuals and diluted the rare Bt resistant genes produced in Bt cotton field by mating with the few resistant moths and originated heterozygote progeny that was easy to be killed by high dosage of Bt toxin from transgenic plants; not only the enough planting acreage of C4 crops was needed, but also the planting time should be planned considering the synchronization of development process of cotton bollworm from C3 and C4 resource.