This case report illustrates the change process of one inmate participant in a music therapy group. The case was selected from a research project of group music therapy for offenders. The participant, Hui, a male inmate, participated in 20 biweekly sessions of group music therapy for male adult inmates in a Chinese prison. Three methods, music improvisation, music and imagery, and song writing, were used in the therapy. Music therapy led Hui (pseudonym) to change from a defensive isolated man into a person who was able to experience and share his emotions with other group members through music. In this case report, we detail the transformative process of Hui and elaborate the music therapist's reflection from theoretical and cultural perspectives. Later, the theoretical issues related to treatment in offender rehabilitation context are discussed in-depth. By making the intervention process transparent, we hope this case report can help readers to understand how the effects of music therapy on decreasing anxiety and depression and improving self-esteem that have been found in research may have taken place in the intervention.