We aimed in this study to determine the cardiac reserve in preterm infants with phonocardiogram. One hundred and fifty-four preterm infants participated in this study. The ratio of the first heart sound to the second heart sound (S1/S2), the ratio of diastolic to systolic duration (D/S) and the cardiac contractility change trend after stimulation (CCCTS) were measured in all infants. The preterm neonatal S1/S2 decreased with increasing gestational age, but the differences between each gestational stage were not significant (p > 0.05), while the D/S significantly increased with increasing gestational age (p < 0.05). After crying induced by vaccination, the D/S was significantly lower than that in quiet state at each gestational stage (p < 0.05). The CCCTS increased with increasing gestational weeks, but the differences between each gestational stage were not significant (p > 0.05).