Background: There has been a dramatic increase in internal migrant workers in China over recent decades, and there is a recent concern of poor mental health particularly amongst younger or "new generation" migrants who were born in 1980 or later.

Methods: A cross-sectional study was conducted in Guangzhou city between May and July in 2012. Mental wellbeing was measured using the World Health Organization Five-item Well-Being Index Scale and the 36 Item Short Form Health Survey mental health scale. Linear and logistic regression models were used to investigate the differences between migrant workers and their urban counterparts and between younger and older migrants.

Results: Migrant workers (n = 914) showed a small but significant advantage in mental wellbeing compared to their urban counterparts (n = 814). There was some evidence for age modification effect (p for interaction = 0.055-0.095); better mental wellbeing in migrants than urbanites were mainly seen in the older compared to the younger group, and the difference attenuated somewhat after controlling for income satisfaction. Older migrants showed better mental health than younger migrants. Factors that were independently associated with poor mental health in migrants included being male, longer working hours, and income dissatisfaction, whilst older age, factory job, high income, and increased use of social support resources were associated with reduced risk.

Conclusions: Efforts to promote mental health amongst migrant workers may be usefully targeted on younger migrants and include measures aimed to improve working conditions, strengthen the social support network, and address age-specific needs.