Though some models of emotion contend that happiness and sadness are mutually exclusive in experience, a considerable number of studies suggest that mixed emotions do exist. In other words, people can feel happy and sad at the same time in emotionally complex situations. In the field of developmental psychology, mixed emotions are defined as the emotional state with two different or opposite-valence emotions in a same situation. The existence of mixed emotions is based on the same situation or condition. Thus, multiple experiences of emotions can co-occur. Several researchers such as Harter & Buddin, Harris and Larson et al., contend that understanding and experience of mixed emotions are different from each other. Large quantity research has shown that children develop a better conceptual understanding of mixed emotions. Researcher assessed the time course of mixed emotions by asking children whether they felt opposite valence emotions ‘‘at the same time, or first one and then the other.’’ Most 4- and 5-year-olds thought that the protagonist would merely feel happy or sad and rejected the notion that the protagonist would feel both happy and sad. Most 7- to 8-year-olds and virtually all 10- to 11-year-olds, however, believed that the story’s protagonist would have mixed emotions. In sum, children’s conceptual understanding of mixed emotions develops with age. Yet, little research has examined children’s actual experience of mixed emotions. Recent findings show that older children are more likely than younger children to report experiencing mixed emotions. These results suggest that in addition to having a better conceptual understanding of mixed emotions, older children are more likely than younger children to actually experience mixed emotions in emotionally complex situations. Language is one of the most important factors which affect the development of mixed emotions. As the vector of cognitive representation and a tool for social interaction, the development of language improves the ability of comprehension, which makes children get more opportunities to communicate emotions with each other and accordingly enhance their understanding of mixed emotions. Moreover, the Secure attachment between infant and mother as well as the Social and economic status of families are also the influence factors of mixed emotions. The discussion about mixed emotions stimulate a considerable amount of research, but the paradigm is consistent with each other. The method of clinical interview is applied to most of the studies, however, is different in material and its representation, objective’s reaction and scoring. In the future, it is necessary to break through in the paradigm and apply the implicit measurement or the method of cognitive neuroscience to explore the development of mixed emotions. Meanwhile, most of studies examined the development of mixed emotions within an important but thin slice of the life span. The developmental trajectory of mixed emotions may extend far beyond childhood and even adolescence. Thus, developmental changes in the understanding, experience, and acceptance of emotional contradiction may play out from the beginning of life to the end. So we need to expand the span of age in our studies and research the developmental trait of mixed emotions from both level and pattern.