Considering a two-tier supply chain composed of one supplier and one retailer, a three-stage game model is constructed to study the impact of supplier encroachment on Retailer's order quantity and supplier's inventory decision under asymmetric information. Through designing the market size threshold and investigating the market size, the profit of suppliers and retailers in each stage is obtained. After comparing the profits of suppliers and retailers in three stages, it is found that when the profits of the latter stage are larger than that of the former stage, the members of the supply chain will adjust the order quantity and inventory quantity, so as to get the scope of order quantity and supplier inventory quantity of retailers in each stage. The results show that supplier encroachment can improve the profit of supply chain within a certain range of retailer orders and supplier inventory.