COVID-19 lockdown has posed unique challenges to postpartum women, but its association with postpartum depression is not well understood in the Global South. This study aims to evaluate the association between COVID-19 lockdown and postpartum depression in rural areas of western China. A multi-stage random cluster sampling method was used to select a cohort of pregnant and postpartum women with infants aged 0-6 months. We conducted an in-person survey before the COVID-19 lockdown and a phone survey right after the lockdown ended. We used multivariate regression models to evaluate the association between lockdown and postpartum depression. Subgroup analysis was performed to explore the role of social support. The overall prevalence of postpartum depression was 13.3%. Postpartum women who experienced the lockdown were less likely to be depressed than those who did not (adjusted odds ratio (aOR) = .43, 95% confidence interval (CI) = [.27, .70]). Lockdown was negatively associated with postpartum depression among postpartum women with low level of social support (aOR = .30, 95% CI = [.18, .51]). COVID-19 lockdown was associated with lower likelihood of postpartum depression, potentially due to increased support from family. Future research is needed to explore targeted interventions to prevent postpartum depression among women from migrant worker families in rural China.