Online sales and green investment are two important ways to improve the sustainability of the tourism supply chain under the pandemic. This research constructs an omni-channel tourism supply chain consists of two tourism service providers (i.e., TSPs) and one online travel agent (i.e., OTA). Tourism products are sold to consumers through three online sales channels, including two free channels and one bundled channel. The bundled channel is operated by the OTA, and the free channels can be operated by the TSPs or the OTA. Depending on the free channels operators, we depict four online selling models, i.e., the model where two free channels are operated by the two TSPs (TT model), the models where only one free channel is operated by one TSP (TO and OT models), and the model where two free channels are operated by the OTA (i.e., OO model). Through comparing the four models, we find that if the TSPs are responsible for green investment, model TT is the most profitable and greenest model compared to the other three models when the positive externality of greenness is high. Otherwise if the OTA is responsible for green investment, the profit and greening level of the tourism supply chain in model TT become the lowest. This study provides managerial insights for tourism practitioners to rationally operate tourism supply chain from three aspects: joint decisions of operations and greenness, channel structure design, and greening level improvement.