Abstract: Urgent order insertion frequently occurs in the production process when the wafer manufacturing mode is changed to multi-variety and small batch. Such an unreasonable scheduling strategy reduces the production efficiency of cluster tools. Thus, in this study, we examine the scheduling problem of single-arm cluster tools faced with urgent order insertion under the constraints of wafer residence time so as to improve the production flexibility of cluster tools. For this, we first analyze the processing process of the urgent insertion order. Then, we propose the scheduling rules of the robot for single-arm cluster tools to realize the scheduling of the processing process. The schedulability judgment conditions are derived for two kinds of machining states caused by different machining parameters, respectively. Accordingly, the mathematical analytical formula for solving the waiting time of the robot is derived. Finally, an optimal scheduling algorithm is proposed for cluster tools in the face of urgent order insertion, and the effectiveness of the scheduling method is verified by an example and comparison analysis.