Alexithymia has been studied with its impact on problematic mobile phone use. However, none of these studies has examined the roles of mindfulness facets in this relationship. To address this issue, a total of 901 Chinese undergraduate students participated in this study and were tested with questionnaires for measuring their levels of alexithymia, problematic mobile phone use, and mindfulness facets. A moderated mediation model was constructed to examine the roles of mindfulness facets in the association between alexithymia and problematic mobile phone use. The results showed that after controlling for age and sex, alexithymia positively predicted problematic mobile phone use both directly (β = 0.157, p < 0.001) and indirectly via mindfulness facet of acting with awareness (16 th , 50 th , and 84 th percentiles of observing were −5.371, β = 0.019; −0.371, β = 0.216; and 4.629, β = 0.242, respectively, and the 95% confidence intervals were 0.142 to 0.246; 0.167 to 0.269; and 0.186 to 0.3, respectively). The facet of acting with awareness partially mediated this relationship in a negative way. Further, the facet of observing moderated the path between alexithymia and the facet of acting with awareness: with a higher level of observing, the negative association between alexithymia and acting with awareness became more negative (alexithymia × observing, β = −0.006, p = 0.001, 95% confidence interval −0.01 to −0.003). The current study advanced our understanding of the mechanism underlying the connection between alexithymia and problematic mobile phone use and helped to investigate how mindfulness skills benefited the individuals.