The specific heat of single-crystal PrMnO3 was investigated from 2 to 200 K under different magnetic fields up to 8 T. A Schottky-like anomaly observed at low temperature was gradually shifted to higher temperatures by magnetic fields. The first four singlets of the Pr3+ 3H4 ground multiplet in PrMnO3 are given for the first time by fitting the specific heat of Pr3+ ions below 40 K under zero field. By analysing the field dependence of the first singlet of Pr3+ ions, the Pr-Mn exchange field is found to be negligible, which is consistent with the magnetic anisotropy of Pr3+ ions revealed in the magnetic measurement. At TN, the cooperative antiferromagnetic ordering of Mn3+ spins shows up as λ-shaped anomaly, which is lowered and broadened in magnetic fields. The magnetic entropy near TN is estimated by subtracting the contributions to specific heat from Pr3+ ions and lattice vibrations. It was found that the fraction of entropy above TN in the total entropy increases with the fields due to the enhancement of spin fluctuations by magnetic field.