E-commerce platforms provide online sales channels for merchants and generate various forms of online advertising services and a cooperation ecosystem. This paper constructs a dynamic game model between the platform and the merchant based on core marketing elements, such as advertising investment and promotion effort, and discusses the cooperative advertising mode and its coordination with joint promotion. The dynamic cooperative advertising strategies of the two parties without and with joint promotion are discussed, and the applicability scenarios of direct and indirect cooperative advertising models are clearly distinguished. The research findings include: 1) Under the direct cooperative advertising model, the platform adopts a “buy more, get more free” support program. Under the indirect cooperative advertising model, the platform’s optimal advertising cost-sharing ratio is influenced only by product and platform characteristics, not by promotional factors. 2) Without joint promotion, the platform’s optimal choice of cooperative advertising mode decision is influenced by the product’s gross profit. The higher the gross profit, the more favorable the platform’s preference for the direct cooperative advertising mode. With joint promotion, the platform’s optimal choice of advertising mode decision is also affected by the merchant's promotional efficiency. The higher the merchant’s promotional efficiency, the more favorable the platform’s preference for the indirect cooperative advertising mode. 3) For low-margin products, the platform increases joint promotion investment. Whereas, for high-margin products, the platform decreases this investment. 4) Joint promotion can form a synergy effect with advertising investment, which can not only reduce the advertising investment of the platform and merchants, but also effectively increase product sales. These findings provide a theoretical basis and managerial insights for platforms and merchants in formulating cooperative advertising strategies, which can help optimize the combination of promotional and advertising inputs in different market environments, thus enhancing overall marketing effectiveness.