Background: As a higher-risk group of postpartum depression (PPD), rural to urban floating women urgently require effective and accessible mental health care after childbirth to prevent PPD. Even though there were various interventions, only a small number of women have sought professional help to reduce their depressive symptoms after childbirth, suggesting the need for an innovative intervention delivery to overcome women’s help-seeking barriers. Online and offline (OTO) interventions, which combine face-to-face and internet-based interventions, provide apparent benefits. As a result, the protocol for a randomized controlled study (RCT) was designed to examine the effectiveness and acceptability of OTO intervention on psychosocial outcomes for Chinese rural-to-urban floating women including the reduction of PPD symptoms and PPD stigma, and the improvement of social support and quality of life. Methods: A double blind, multicenter, RCT will be used and a total of 226 participants will be recruited. The OTO intervention called the “Hi, Mom” program will integrate two face-to-face consulting sessions with online sessions comprising an information module, a communication module, an ask-the-expert module, and a peer story module over a period of three months. The control group will receive routine postpartum care. Outcome measures including PPD symptoms, PPD stigma, social support, quality of life, mother–child bonding, and satisfaction with health care received will be conducted at baseline, postintervention, and three-month follow-up. Results and Discussion: If the intervention is effective, it will provide a convenient and effective intervention program on postpartum mental well-being for rural-to-urban floating women. As the first study to test the effects of an OTO intervention for the prevention of PPD in China, the outcomes gained from this study will provide evidence-based knowledge for clinical practice on PPD prevention based on online and offline health technologies. Moreover, it could be used to plan a culturally appropriate OTO intervention for migrant mothers from different countries.