Left-behind children refer to children whose parents or one of them works outside the home for six months or more every year, or children under 16 who are taken care of by their parents, grandparents or others [1]. In recent years, with the increase of floating population in China, the number of left-behind children has also increased accordingly, and a large number of documents report that the mental health problems of left-behind children are very prominent. Li Shiling et al. [2] found that the mental health level of rural left-behind children in Yongchuan District of Chongqing is lower, and the problem is more serious than that of non-left-behind children. Li Keng and others [3] found that left-behind children are more sensitive, anxious, fearful and paranoid than non-left-behind students. The survey results of Fan et al. [4] also show that the learning disabilities and personality defects of rural students are significantly higher than those of urban students. There are many factors that cause psychological problems: First, children's personal factors. Gao [5] found that there are differences in self-blame, learning anxiety and terror among left-behind children of different sexes; Followed by the guardian factor, Zhao [6] found that the guardian's concern for children is also the main factor affecting their mental health when investigating the mental health of left-behind children in rural areas of Ningxia; Thirdly, school factors, Zhang Liansheng [7] found that boarding will affect the mental health of left-behind children in rural areas of Hubei. Jia Ming [8] believes that the future research direction will be carried out from four aspects; First of all, the inclusion criteria of left-behind children will be unified and the results will be more comparable; Secondly, the research tools will use qualitative interviews and quantitative surveys instead of questionnaires; Third, the location of the survey is not limited to the region, but will conduct a national survey; Finally, there will be more and more large sample surveys, and the research results will become more and more true and accurate.

2. Rural children

Nowadays, the lack of mental health education for rural primary school students is very worrying, and all sectors of society should pay more attention to guide them to grow up actively and healthily. Liu Jianlin and others [9] think that rural pupils are more likely to have psychological problems and bad behavior habits than urban students, such as lying, loss, weariness of learning, violence and other psychological problems. There are three reasons: first, students who have difficulties in life, live in single-parent families or follow the elderly, and these children lack discipline, which leads to their poor binding force and self-control, and are easy to be emotional and sloppy [10]. Second, the parents of primary school students in rural areas are generally not well educated. Some parents only care about their children's food and clothing, and seldom realize the importance of supervising their children's study. Some parents often reprimand their children for lack of emotional communication; Some parents overindulge [1 1]. Third, many rural schools still take the enrollment rate as the highest goal, and students' psychological problems and bad behaviors and habits in daily life cannot be discovered in time, which leads to the increasingly serious psychological problems of students [9].

3. Single parent families

With the rising divorce rate in China, more and more single-parent families appear. Children in single-parent families are influenced by the lack of affection, family guidance and public opinion.