Purpose: The purpose of this paper is to build a moderate mediation model to delineate the effects of leader humility on employee constructive voice behavior based on conservation of resources theory and crossover of resources model. Specifically, when a leader behaves with humility, the followers will be more likely to feel they receive psychological resources from their interactions with the leader (i.e. relational energy), and thus engage in more constructive voice behavior. In addition, this energizing effect only occurs when the leader is perceived as having higher apparent sincerity by their subordinates. Design/methodology/approach: The research hypotheses of this study were empirically tested using multi-timepoint and multi-source (i.e. supervisors and subordinates) survey data in China. Study 1, based on data from 449 subordinates and 88 immediate supervisors, was conducted to test the proposed mediation effect. Study 2, based on data from 185 subordinates and 50 immediate supervisors, was conducted to replicate the findings of Study 1 and test the integrated model. Findings: The results of Study 1 support the proposed mediation effect that leader humility positively predicts followers’ constructive voice behavior via boosting followers’ relational energy. The results of Study 2 replicate the findings of Study 1 and further indicate that leader humility is only positively related to perceived relational energy when a leader is perceived as having higher apparent sincerity by their subordinates. Practical implications: This paper provides detailed instructions for business practitioners. First, given that employee constructive voice behavior is related to employee relational energy and is beneficial to organizations, leaders are encouraged to behave with humility when interacting with their subordinates. Second, from the perspective of human resource management, well-designed training programs can be used to help leaders to develop leader humility. Third, the findings of this paper call attention to a potential risk for humble leaders. Organizations should educate leaders on the potential negative consequences of false humility and encourage leader humility that will appear in leaders’ sincerity. Originality/value: Overall, drawing on conservation of resources theory and crossover of resources model, this paper reveals that boosting relational energy is a mechanism via which humble leaders can shape employees’ voice behavior. Second, by examining the moderation effect of apparent sincerity of leaders from followers’ perspective, this paper suggests an actor–recipient perspective to identify the boundaries of the energizing mechanism. Third, the findings of this paper add to the knowledge on voice research by highlighting an additional source of energy for employee constructive voice behavior.