Previous studies on interrupting have paid little attention to its dynamic generating mechanism. This paper, taking assistance-type cooperative interrupting as the research object, has made a comprehensive interpretation of how it is generated from the perspective of relevance theory and politeness principle. It is found that assistance-type cooperative interrupting is a verbally communicative behavior in which the interrupter, governed by the politeness principle, assists the interrupted person in conveying communicative information and intention based on the assumed understanding inferred. This process entails two pragmatic manifestations: 1) the awareness of participating, namely, confirming the conversational topic initiated by the interrupted person; 2) the awareness of caring for others’ needs, namely, satisfying the transactional motivations of the interrupted person. This study deepens the understanding of interrupting, which can provide inspiration for future research.