Increasing global competition has forced many manufacturing enterprises (outsourcers) to focus on their core competences and outsource low value-added parts machining activities to suppliers to increase product quality and productivity as well as cut cost. While outsourcers or suppliers coordinate their decisions with partners for parts machining outsourcing, the present problem is how to timely achieve the most beneficial portfolio with the goal of gaining mutual benefit in a game structure. Based on the investigation conducted in Weinan National High-New Technology Zone of China, this article identifies several typical outsourcer–supplier Stackelberg game models for the order coordination of parts machining outsourcing and investigates one scenario of them in detail. Then, to solve the established bi-level programming model corresponding to the Stackelberg game, a solution procedure based on modified imperialist competitive algorithm is proposed. Finally, a case from a printing machinery enterprise is analyzed to validate the proposed model. This research is expected to improve the quality and effectiveness of coordination decision-making in parts machining outsourcing.