With the aid of differential game, the long-term joint emission reductions and low-carbon promotion problems of the supply chain system consisted of a single manufacturer and two retailers are studied. In the case that the consumer demand is affected by emission reductions of products and the retailers’ low-carbon promotion, three differential game models are constructed. It is found that the cost-sharing contract can achieve Pareto improvement of manufacturers’ and retailers’ profits under certain conditions. With the increase of competition, the profits of the manufacturer and supply chain system increase while the retailers’ profits decrease. Finally, the model is analyzed through an example.