Methadone maintenance treatment (MMT) has become an important modality of substitution treatment for opioid addicts in China since 2006. However, data are limited regarding the change in sexual function from heroin use to MMT and the influence of sexual dysfunction (SD) during MMT on patient's life and treatment. Face-to-face in-depth interviews were conducted with 13 male and 14 female MMT patients, five of their partners, and three clinicians. The interviews took place in four MMT clinics in Guangdong Province between August 2010 and February 2011. The patients and their partners were asked separately for their perceptions of patient's sexual function during MMT, and the influence of SD on personal/family life and treatment. The main SD problems patients perceived were libido inhibition and decreased sexual pleasure. Methadone was thought to have a stronger inhibition effect on sexual desire than heroin. SD decreased quality of patient's sexual life and damaged intimate relationships. There was a gender difference in coping with SD. Men generally tended to refuse, escape, or alienate their partners. Women tended to hide sexual listlessness, endure sexual activity and tried to satisfy their partners. SD might increase risk of voluntary dropout from treatment and illicit drug use during treatment. Patients with SD did not get any effective therapy from clinicians and they also lacked skills on coping with SD-related problems. Sexual dysfunction prevented patients from reconstructing a normal intimate relationship, and affected stability of maintenance treatment. Response to patient's SD and SD-related problems from clinicians was inadequate. There is a need to develop a clinical guide to deal with both SD itself and SD-related problems.