This paper considers the cooperation strategies in an Online Selling Low-Carbon Supply Chain (OSLCSC). The manufacturer in OSLCSC establishes an online retailing channel based on a co-operative agency sales format. Three possible online collaboration strategies are considered. We analyze the final cooperative equilibrium and find that cooperation between the platform and manufacturer can promote the dual development of environmental benefit and over-all supply chain profit. But from an individual benefit perspective, the platform and manufacturer do not always have motivation to cooperate with each other, and it related to the revenue-sharing rate and market competition intensity. Furthermore, we reveal that non-cooperation strategy does not always lead to the worst outcome, and in some cases, cooperation between the retailer and platform will result in the lowest entire supply chain performance. Lastly, the results show that each cooperative strategy may be the ultimate equilibrium, and the three players in OSLCSC could achieve a Pareto optimal under the different cooperation strategies.