Background: Chinese mothers of preterm infants often face obstacles to breastfeeding and commonly experience prolonged maternal-infant separation when their high-risk infants are hospitalized in a Neonatal Intensive Care Unit (NICU). This separation hinders mother-infant attachment and the establishment of breastfeeding. Currently, little is known about Chinese mothers’ experiences breastfeeding their preterm infants, or their support needs. The aim of this study was to develop an understanding of mothers’ experiences breastfeeding a hospitalized preterm infant and the support needed to establish a milk supply during the period separation from their infants. Methods: A qualitative descriptive study was conducted in Beijing in 2017. A total of 11 Chinese mothers were individually interviewed while separated from their infants. The interviews were audio-recorded and transcribed verbatim. A thematic analysis involving a seven-step protocol identified key themes. Results: Mothers of preterm infants reported physically and mentally challenging breastfeeding experiences during the period they were separated from their babies. They viewed expressing breast milk as integral to their maternal role, even though some found expressing breastmilk exhausting. With little professional support available, the mothers depended upon nonprofessionals to establish breastfeeding. Conclusions: The study identified the difficulties mothers experienced establishing a milk supply while separated from their preterm infants, and the importance of access to health professional support.