Social anxiety is becoming a worldwide mental health problem and in-service staffs are one of several subgroups most vulnerable to this problem. According to the causality model, the lack of social support can lead to social anxiety. Empirical studies have found that perceived less social support may lead to higher social anxiety. But most of the studies pay more attention to individual factors such as personality to explore the influence mechanism on social anxiety. Far too less attention was paid to the point of interpersonal interaction. Rumination, passively and repetitively dwelling on and questioning negative feelings in response to distress, is a risky factor for the development of psychopathology (Hilt & Pollak, 2012). Social undermining is defined in a work context as behavior intended to hinder, over time, a worker’s ability to establish and maintain positive interpersonal relationships, work-related success, and favorable reputation (Duffy, Ganster & Pagon, 2002). In real life, individuals are affected by both social support and social undermining. Social undermining weakens the role of social support by adding pressures and lowering self-efficacy. The recent study has brought in social undermining and rumination to explore the influential mechanism of the perceived social support on social anxiety and aim to provide the basis for improving and alleviating the extension of social anxiety. Questionnaires are used as methodology to test the hypotheses in this essay. A total of 471 in-service staff (259 males & 212 females), aged from 24 to 55, participated in the study. This study has translated the social undermining scale and used PSSS, RRS, IAS. Bootstrap of Mplus is used as the analytic method to analyze and process the data. The result indicated that: (1) Rumination mediates the relation of perceive social support to social anxiety. Perceived social support predict rumination(β = -.256，P = 0.008) and rumination predicted social anxiety. (2) Social undermining moderate the effect of perceived social support to rumination. The interaction terms(perceived social support x supervisor social undermining) predict the rumination explicitly(β = 0.289, P < 0.001) and the interaction terms (perceived social support x coworker social undermining) also predict a rumination obviously (β = 0.289，P < 0.001). When the levels of social undermining are low, the relation of perceived social support to social anxiety is crucial, while the relation is not crucial with a high social undermining. This study shows a new theoretical and practical view to clarify the social anxiety. It has argued that the findings enrich the causality model by confirming the mediated role of rumination in the relation of perceived social support to social anxiety. This study also confirms that social undermining moderate the impact of perceived social support on rumination. The participants of this study, however, from a variety of professional group, there may be heterogeneous. But this study did not focus on the difference and assume them as homogeneous. More studies should use diverse methods and create diverse models to explore how a professional difference could lead to a difference of the social anxiety.