Abstract: As cotton plant areas decrease sharply at the Yellow River Valley and to relive the stress of competing for land between grain crops and cotton, the effect of wheat-cotton double cropping system on cotton canopy light capture and distribution of cotton bolls had been studied. The completely random design was used with three treatments of cotton monoculture system, wheat-cotton double cropping system, cotton relay wheat cropping system and each repeated triply. Incident and reflected light of each treatment was measured by light quantum sensor, meanwhile cotton growth period and plant mapping were determined. And the results were as followings, 1) there were significant differences in cotton growth period between three treatments, comparing with monoculture cotton, the duration from cotton seedling stage to cotton square stage for cotton relay wheat cropping system was 10 days and 7 days shorter in 2016 and 2017, respectively, however, duration from flowering stage to boll-opening stage was 15 days and 17 days longer, respectively. 2) The capacity of light interception was highest for cotton in monoculture system, for example, 71.4% and 78.6% higher light was captured by cotton of monoculture system than of wheat-cotton double cropping system and cotton relay wheat inter-cropping system, respectively. 3) The number of cotton boll for monoculture cotton and wheat-cotton double cropping system was decreased from middle sympodial branches to upper and lower part, while it was the most at the bottom sympodial branches for cotton relay wheat cropping system. All in all, wheat-cotton double cropping system influenced cotton canopy light interception by affecting cotton growth period, finally different yield distribution being obtained.