Focusing on a two-echelon supply chain consisting of a fresh agricultural product supplier and a retailer, this study integrates presale spillover effects and advertising investment into a unified analytical framework. It constructs supply chain decision-making models under centralized and decentralized scenarios, respectively, and employs contracts for optimization. The findings are as follows. The profit of the centralized supply chain is superior to that of the decentralized one, with higher presale prices, advertising intensity, and fresh-keeping effort levels. The presale spillover effect significantly increases the levels of decision variables across all stages by facilitating the conversion of potential consumers, whereas the cost coefficients of advertising and freshness preservation exert an inhibitory effect on these variables. A two-way revenue-sharing and costsharing contract can achieve Pareto optimization for all parties in the supply chain under appropriate proportions. This research reveals the intrinsic mechanism of synergistic decision-making between advertising placement and presales, providing a theoretical basis for optimizing advertising resource allocation and improving market conversion efficiency in fresh agricultural product supply chains.