Parents who are far away from their children in their hometowns equip their children with mobile phones to effectively educate and supervise them as a substitute for face-to-face communication. However, left-behind children have weak self-control, and the supervision of their grandparents is somewhat ineffective, leading to frequent cases of mobile phone addiction. Regarding the use of mobile phones, does it have a positive or negative impact on their mental health? What roles do parent-child relationships and family education methods play in this? This study uses data from the 2020 China Family Panel Studies, selects samples of left-behind families, measures the core variables with corresponding items, and uses stepwise regression to study the impact of mobile phone usage time on the mental health of left-behind children and the mediating effect of parent-child relationships. Group regression is used to test the moderating effect of family upbringing methods. The results show that the direct impact of mobile phone usage time on the mental health of left-behind children is not significant, but the indirect effect is significant. That is, mobile phone usage time negatively affects parent-child relationships, while parent-child relationships positively affect mental health. The impact of mobile phone usage time on the mental health of left-behind children is moderated by family upbringing methods. In permissive parenting styles, mobile phone usage time has a significant negative impact on mental health, while in democratic parenting styles, it has a significant positive impact. In authoritarian and authoritative parenting styles, there is no significant impact. Democratic parenting styles are significantly different from the other three.