With the prevalence of shopping festivals launched by e-commerce platforms, online retailers and customers are gradually showing signs of fatigue. In order to enrich the promotion strategies of the shopping festivals, break the bottleneck, and achieve mutual benefit, this paper proposes three promotional models based on platform-based promotional effort, namely, the joint promotion model with no promotional effort, the joint promotion model with only the platform putting in the promotional effort, and the joint promotion model with the platform and the e-retail sharing the promotional effort. The study shows that the platform and the online retailer's promotion strategies are influenced by the commission factor and the promotional effort cost-sharing factor. When the latter factor is appropriate, it is more beneficial for the e-retailer to choose the cost-sharing strategy of promotional effort. In addition, the platform investing in promotional effort not only improves its profits, but also helps maximize the profits of the e-retailer. The study further finds that consumers acquire the maximum discounts when there is no promotional effort. As the promotional effort is invested, the discount declines and the demand rises instead.