Nutritive sucking is the approach of obtaining primary food for newborns within six months. However, completing the precise coordination of sucking-swallowing-respiratory actions under oral feeding is usually difficult for preterm infants and may affect the body and neural development of the premature infant. For this reason, how to obtain the quantitatively physiological information related to oral feeding is useful for physicians making more accurate clinical diagnoses for oral feeding disorders. However, there is still lack of monitoring systems for evaluating the feeding skill of infants quantitatively and objectively. The assessment of the feeding skill or the diagnosis of oral-feeding disorders still depends on the clinical experience of medical staffs. In this study, a novel wireless monitoring system is proposed to evaluate the feeding behavior of infants quantitatively and objectively and analyze the characteristics of these actions under oral feeding. Here, a specific feeding bottle and an optical probe are also designed and successfully applied to non-invasively monitor the sucking and respiratory actions, and a microphone is also applied to monitor the swallowing action. From the experimental results, the sucking and swallowing rate increases and the respiratory rate decreases with the increase of the infant age. Moreover, the duration increases and the interval of sucking-action sequence decreases with the increase of the infant age respectively. The proposed system successfully assesses the feeding behavior of preterm infants and may provide an objective evaluation tool for oral feeding ability and identifying the risk of infant developmental delay in the future.