This paper explores the impact of retailers' corporate social responsibility (CSR) and manufacturers' overconfidence on manufacturers' carbon reduction in sustainable supply chains. We analyze the profits of manufacturers and retailers under different scenarios and explore the social welfare and environmental impacts under CSR. Our results suggest that retailers' CSR and manufacturers' overconfidence contribute positively to promoting carbon mitigation and reducing environmental impacts under certain conditions. However, with increasing CSR and manufacturer overconfidence levels, manufacturers are more likely to lead to worse environmental impacts and carbon emission reduction. In addition, we show that when the manufacturer's overconfidence level is high, manufacturers and retailers are more profitable and contribute to carbon emission reductions in the manufacturer without overconfidence (retailer without CSR) scenario. Moreover, we find that firms have the higher potential to capture optimal overall social welfare in the presence of retailers with CSR and manufacturer overconfidence.