SEPARATION ANXIETY disorder is one of the issues and problems that lead to academic failure and huge losses for the parents and society. This is a qualitative research on SEPARATION ANXIETY disorder in preschool children. The present research method was qualitative with a grounded theory approach based on Charmaz constructivist model. The population included mothers of children with SEPARATION ANXIETY disorder in Tehran in 2020, from whom 10 mothers were selected by homogeneous purposeful sampling until saturation. The semistructured interview method was used to collect data and the data were analyzed with the MAXQDA software 2020 version. 370 primary codes, 4 themes including the position of strategic management application, the position of the determining father-affected mother, the effective position of the underlying factors, and the position of the child's sensitivity in infancy and childhood and showed a central category of early sensitivity to interactions and environment. It can be concluded that inadequate understanding of interactions and dysfunctional parenting strategies are effective in SEPARATION ANXIETY, and therefore to prevent this disorder, parental educational interventions and the involvement of fathers in parenting are recommended.