Abstract Children's emotional understanding competence is an important component of their emotional socialization that is the foundation of emotional communication and social interaction. It is also a good indication of their social-emotional development and social adjustment. Children who have high emotional understanding competence are more likely to recognize others' emotions and predict others' behaviors, thus are more accepted by other children. Among the factors that influence children's emotional understanding competence, mothers' reactions to their children's negative emotions play an important role. Mothers accept and talk about children's negative emotions, which facilitates children' acceptance of negative emotions. Mothers' emotional regulation also influences their ways of coping with emotional events and their attitudes and reactions to children's negative emotions, which eventually affect children's emotional understanding competence. In the present study, we focused on the relationship among the mothers' emotional regulation mode, maternal reactions to children's negative emotions, and children's emotional understanding competence. The aims of the present study were (a) to examine whether mothers' emotion regulation mode significantly predicts children's emotion understanding competence and mothers' reactions to their children's negative emotions, and (b) to investigate the mediating role of maternal reactions to their children's negative emotions between mothers' emotional regulation mode and children's emotional understanding competence. The present study used emotional comprehension task to examine 258 children's (aged from 3-5 years old, mean age 54.92±9.73 months，49.4％boys) emotion understanding competence, their mothers completed emotional regulation questionnaire (ERQ) and coping with children's negative emotions (CCNES) questionnaire. The results indicated that the level of children's emotional understanding competence increased with age. The gender difference was significant, with girls performing better than boys in emotional comprehension task (p<.01). Mothers scored highest in the mode of emotion-focused and problem-focused, and scored lowest in punishment reaction (p<.01). Mothers used more supportive reactions than non-supportive reactions when coping with their children's negative emotions. Mothers displayed more punishment and non-supportive reactions to boys' negative emotions, but more emotion-focused and supportive reactions to girls’ negative emotions (p<.01). Mothers' emotion regulation mode (positive-rumination, positive-revealing and negative-reappraisal) significantly predicted children's emotional understanding competence, which remained significant after adding maternal supportive reactions (encouragement expression, emotion-focused and problem-focused). Maternal supportive reactions, especially the encouragement expression, to children's negative emotion played a mediating role between maternal emotional regulation mode and children's emotional understanding competence. In summary, mothers showed more emotion-focused, problem-focused reactions to their children's negative emotions. Mothers displayed more punishment and non-supportive reactions to their sons' negative emotions, but more emotion-focused and supportive reactions to their daughters' negative emotions, which may lead to girls performing better than boys in emotional comprehension task. Maternal supportive reactions, especially the encouragement expression reactions, to children's negative emotions had a mediating role between maternal emotion regulation mode and children's emotional understanding competence, which suggests that, mothers' supportive reactions especially encouraging their children to express their bad feeling may benefit their children's emotion understanding competence when children experience negative emotions.