Thermodynamic quantities of the Einstein solid model are studied via the microcanonical approach.We point out that the number of phonos in Einstein solid model cannot be defined independently and thus the chemical potential of phonos cannot be defined in the microcanonical ensemble. The chemical potential of phonos can be calculated only if the number of phonons can fluctuate. And we prove that the chemical potential of phonos in Einstein solid model strictly equals to zero in other ensembles.