Women with prior gestational diabetes mellitus (GDM) are at a higher risk of type 2 diabetes and other health issues after delivery. They may have a lower quality of life (QoL), experience more medical-related stress, and need more support than those without it. This study aimed to examine the six-month efficacy of an intensive lifestyle modification program on perceived stress, social support, and QoL among women with prior GDM in rural China. A total of 320 women with prior GDM were randomly assigned to an intervention group (n = 160) and a control group (n = 160). Participants in the intervention group received an intensive lifestyle modification (ILSM) program, including a series of six biweekly face-to-face sessions and five biweekly phone sessions delivered by trained local health workers. The control group received the usual care. Data about perceived stress, social support, QoL, and HbA1c were collected at baseline, at three months, and at six-month follow-ups. Generalized estimating equation analysis was used to assess the efficacy of the intervention. There were significant improvements in the psychological domain (β = 0.479 ± 0.153, p = 0.002) and environmental domain (β = 0.462 ± 0.145, p = 0.001) of QoL over six months; there were significant group effects (β = -0.718 ± 0.280, p = 0.010) and time effects (β = 0.453 ± 0.211, p = 0.032) in physiological domain, and there were significant group effects in the social relations domain (β = -0.669 ± 0.321, p = 0.037). The ILSM group had a more pronounced downward trend in HbA1c than the control group (β = -0.050 ± 0.026, p = 0.059). The ILSM program can help women with GDM improve their psychological and environmental domain of QoL. It can be recommended as a form of health promotion for improving QoL among women with prior GDM in rural primary care settings in developing countries.