[Aim]  To clarify the effects of Azotobacter chroococcum inoculation on the target insect-resistance of transgenic Bt cotton. [Methods] We used the seed immersion coating method to infect Bt cotton (Bt) and non-Bt cotton (Yu) seeds with Azotobacter chroococcum (AC) and the sterilized medium (CK). A laboratory test was carried out to study the Bt toxin protein expression and nitrogen metabolism physiology of cottons inoculated with A. chroococcum, and investigate the growth, development, reproduction and food utilization of cotton bollworm, Helicoverpa armigera fed on cottons infected by A. chroococcum. [Results]  The findings indicated that the infection of azotobacter significantly increased the Bt toxin protein expression (+26.73%) and the nitrogen metabolism indexes of cotton, as soluble protein content (Bt: +17.69%; Yu: +6.16%), GPT (Bt: +72.64%; Yu: +60.27%) (P < 0.01), except for the decreasing Free amino acids content (Bt: ﹣0.90%; Yu: ﹣5.80%) and protease activity (Bt: ﹣1.83%; Yu: ﹣4.06%) (P ≤ 0.001). Compared to CK, AC treatment obviously increased the larval life-span (+11.57%), pupal duration (+1.03%), relative consumption rate (+6.07%) and approximate digestibility (+4.17%) of H. armigera (P < 0.05), and reduced the adult longevity (﹣4.09%), pupation rate (﹣46.09%), pupal weight (﹣21.09%), fecundity (﹣59.86%), innate rate of increase (﹣9.31%), relative growth rate (﹣4.26%), efficiency of conversion of digested food (﹣5.82%) and efficiency of conversion of ingested food (﹣5.51%) of H. armigera (P < 0.05). There were opposite trends on the growth, development, reproduction and food utilization of H. armigera fed on non-Bt cotton infected by A. chroococcum compared with CK. [Conclusion]  Overall results significantly depicted that the infection of azotobacter had positive effects on the Bt toxin protein expression and nitrogen utilization of Bt cotton to enhance the target insect-resistance.