This paper analyzes the advertising decisions in a dual-channel supply chain involved one manufacturer and one retailer. A Stackelberg game dominated by the manufacturer is established. The influence of asymmetric demand information and dual-channel on the cooperative advertising decisions is analyzed. The study shows that the retailer will lie about the demand when it has demand information advantages. Under certain conditions, the optimal lied strategy would decrease manufacturer’s profit but increase the profit of the supply chain. Under such circumstances, the manufacturer could not use the incentive contract to encourage the retailer to share demand information. In addition, when the market share proportion between direct sales channel and distribution channel is less than a certain value, the introduction of direct sales channel can be beneficial to both the supply chain participants under optimal advertising strategies.