Long-term imprisonment can cause severe emotional problems, which in turn can trigger behavioral problems, self-harm, and suicide. Mindfulness-based intervention can enhance emotional health. This study investigated the effects of a 6-week mindfulness training program on the emotional health of long-term male Chinese prison inmates. Forty long-term male prisoners completed a pretest and posttest, with 19 in the mindfulness training group and 21 in the waitlist control group. The treatment group showed a significant improvement in mindfulness level, anxiety, depression, tension-anxiety, depression-dejection, anger-hostility, confusion-bewilderment, and total mood disturbance. Implications and limitations of this study were discussed. These results support the use of a mindfulness-based intervention to enhance the emotional health of long-term male prison inmates.