<p>The relationship and mechanism between left-behind experience and adulthood depression remain unclear. This study aims to investigate the effect of left-behind experience and the mediating role of social support networks on adulthood depression. Using the China Labor-force Dynamics Survey in 2018, we examined the relationship between left-behind experience and adulthood depression. The mediation analysis indicated that emotional support and informative support networks mediate the impact of left-behind experience on adulthood depression. These findings demonstrate that childhood left-behind experiences do indeed affect the level of depression in adulthood, and from the social support perspective provide an understanding of the mechanisms underlying this effect. The time-dose effect and heterogeneity effect of left-behind stage also found in this study: longer duration of being left-behind and younger left-behind stage are positively associated with higher levels of adulthood depression, and individuals who experienced being left-behind at a younger stage tend to have a higher depression level.</p>