Small- and medium-sized enterprises (SMEs) in low-carbon supply chains often face financing obstacles for improving carbon emission reduction (CER) level. Armed with unique advantages beyond offline financing methods, online peer-to-peer (P2P) lending can offer financing service for SMEs to alleviate capital constraints. This article investigates the equilibrium strategies under online P2P lending and effects of such a financing scheme in funding low-carbon supply chains through multi-level Stackelberg game models under two possible scenarios: the manufacturer independently bears CER investment cost and the retailer shares the cost with the manufacturer. In the models, a P2P platform first sets a service rate and charges a commission. Under a carbon cap-and-trade policy, a manufacturer borrows capital through the P2P platform and decides CER level. As the follower, a retailer determines a retail price. We find that the P2P platform's financial decision is a double-edged sword in influencing its own profit through commission and loan size. The cost sharing is alarmingly not always helpful for the supply chain finance system and improvement of CER level. Online P2P lending plays a complementary role to bank credits by bringing more social welfare and greater promotion degree of CER level.