There have been rich debates about whether and how mindfulness alters prosocial behaviour. Nevertheless, few empirical studies have touched on how mindfulness training (MT) influences altruistic behaviour under high- and low-cost situations in a real-life scenario. The present study aimed to examine the effect of mindfulness training on altruistic willingness at different cost levels. A total of 41 females participated in our study and were randomly assigned to the MT and control groups. They completed the empathy-altruism task and Five-Facet Mindfulness Questionnaire (FFMQ) before and after an 8-week experimental intervention, during which the MT group attended the Mindfulness-Based Cognitive Therapy (MBCT) programme, while the control group remained as usual. The MT group presented a significant increase in overall FFMQ scores after the 8 weeks of MBCT. However, their willingness to help declined in the low-cost situation at post-test. Further analysis revealed a positive correlation between the increase in the scores of the observing facet and willingness to help in the high-cost situation in the MT group. The changes in describing facet were a negative predictor of the change in empathy in the low-cost situation. Taken together, 8-week MBCT enhanced the level of mindfulness but reduced people's willingness to help in the low-cost situation.