Based on the family systems theory and the parent-child relationship model of parenting stress, a total of 5362 parents of primary school students were investigated by using questionnaires. From the perspective of relationships in the family system, the present study explored the relationship between co-parenting and parenting stress, as well as the mediating role of parental perception of parent-child relationship between the two. The results showed that: 1) Integrity, conflict, and disparagement in co-parenting could significantly predict parenting stress, while integrity had no significant predictive effect on parenting stress. 2) Parent-child intimacy played a partial mediating role between integrity/conflict in co-parenting and parenting stress, while it played a fully mediating role between consistent co-parenting and parenting stress; parent-child conflict partially mediated the relationship between integrity/conflict/disparagement in co-parenting and parenting stress. This study explored the impact mechanism of parenting stress from the perspective of family relationships, which provides a basis for intervention measures such as family therapy. The results emphasize the construction of positive relationships among family members, which aim to provide support for parents by creating a positive family relationship atmosphere and good interaction modes, that can alleviate parenting stress.