Objective To explore the relationship between the medical social support, childbirth readiness and fear of childbirth, so as to provide a basis for developing relevant nursing interventions. Methods A total of 747 primiparas from 10 general hospitals and 3 obstetrics and gynecology hospitals in 7 provinces of China were investigated using the general information questionnaire, the Chinese Mandarin version of the Medical Outcomes Study Social Support Survey, the Childbirth Readiness Scale and the Fear of Birth Scale. Results The primiparas scored （74.43±14.30） points in medical social support, （76.75±10.78） points in childbirth readiness and （5.02±2.42） points in fear of childbirth.Primiparas′ medical social support was positively correlated with childbirth readiness and negatively correlated with fear of childbirth （both P＜0.05）.Childbirth readiness was negatively correlated with fear of childbirth （P＜0.05）.Childbirth readiness partially mediated the relationship between medical social support and fear of childbirth, and the effect ratio was 35.09%. Conclusion Childbirth readiness plays a mediating role between medical social support and fear of childbirth in primiparas.Medical workers should take intervention measures to improve childbirth readiness of primiparas and alleviate fear of childbirth.