Objective To explore the effect of online cognitive-behavioral therapy (CBT) on relief of negative emotions in women with gestational diabetes mellitus (GDM). Methods Ninety women with GDM were selected to participate in an online CBT program, which included 6 weeks of treatment and 3 weeks of maintenance. The effect was evaluated before and immediately after the intervention and at 3rd day after the delivery of the baby using the Pregnancy-Related Anxiety Scale and the Edinburgh Postpartum Depression Scale. Then 14 women who completed the whole intervention received a semi-structured interview, and themes were analyzed using the Colaizzi′s descriptive phenomenological method. Results The anxiety and depression scores had significant differences at different time points (P<0.01 for both). Three themes were identified through the interview, including enhancing positive emotions and improving coping ability, meeting the needs of home health care services, and uncertain factors of online therapy. Conclusion Online CBT can relieve negative emotions of women with GDM and meet their needs of home care services. However, in the future, it is still necessary to optimize the scheme to enhance the treatment effect.