The effects of the motion of atomic mass centre and field mode structure on the dynamics of the field phase in the Jaynes-Cummings model are discussed by means of the Hermitian phase formulas of Pegg and Barnett. The time evolutions of the phase probability and the phase fluctuation, with the motion of atomic mass centre neglected and the motion not neglected, respectively, is derived. Under the two conditions, the dynamics of the field phase are compared. The results show that the motion of atomic mass centre leads to the periodic evolution of the dynamics of the field phase and this period depends on the field mode structure parameter.