[Objectives] Circadian clock is easily traced in the growth, behavior, physiology and other aspects of organisms. To explore the effect of photoperiod and temperature on the diurnal expression of the circadian clock gene cwo in cotton bollworm, Helicoverpa armigera, larvae. [Methods]  The full-length ORF (open reading frame) of cwo cDNA was obtained through transcriptome sequencing. The spatial and temporal expression of cwo and its circadian rhythm of expression under different photoperiod and temperature conditions were analyzed with real-time PCR. [Results]  Sequence analysis showed that the ORF of the cotton bollworm cwo cDNA sequence was 1 335 bp, encoding 444 amino acid residues, and had a predicted protein molecular mass of 49.95 ku. Spatial and temporal expression analysis showed that the cwo gene was more highly expressed in the brain and ventral nerve cord than in other tissues. Transcription of this gene peaked during the hatching stage, the molting period, larval-pupal metamorphosis and 1 d before eclosion. Analysis of the expression of this gene in the head of cotton bollworm larvae indicated that under normal photoperiod conditions the expression of cwo had an obvious circadian rhythm and peaked during the scotophase. Under a short scotophase, or photophase, the timing of cwo expression was advanced, whereas long scotophase or photophase significantly decreased the circadian oscillation of cwo expression. cwo expression was significantly up-regulated in the short term at 38℃, and its rhythm was disturbed, whereas at 18℃ expression was significantly down-regulated during the scotophase and its rhythm was unchanged. [Conclusion]  The circadian expression of the circadian clock gene cwo in larvae of the cotton bollworm is influenced by photoperiod and temperature.