The physical properties, evolution, and fragmentation of massive dense cores (MDCs, ~0.1pc) are fundamental to the understanding of high-mass star formation. We aim at characterizing the temperature, velocity dispersion, and fragmentation of the MDCs in the Cygnus X giant molecular cloud and to investigate their stability and dynamics. We present the Karl G. Jansky Very Large Array (VLA) observations of the NH_3_ (J, K) = (1,1) and (2,2) inversion lines towards 35 MDCs in Cygnus X, from which the temperature and velocity dispersion are calculated. We extract 202 fragments (~0.02pc) from the NH_3_ (1,1) moment-0 maps with the GAUSSCLUMPS algorithm. The stability of the MDCs and their NH_3_ fragments is analyzed through evaluating the corresponding kinetic, gravitational potential, and magnetic energies and the virial parameters. The MDCs in Cygnus X have a typical mean kinetic temperature TK of ~20K. Virial analysis shows that many MDCs are at sub-virialized states, indicating that the kinetic energy is insufficient to support these MDCs against their gravity. The calculated non-thermal velocity dispersions of most MDCs are at transonic to mildly supersonic levels, and the bulk motions only have a minor contribution to the velocity dispersion. As for the NH_3_ fragments, with TK~19K, their non-thermal velocity dispersions are mostly trans- to sub-sonic. Unless there is a strong magnetic field, most NH_3_ fragments are probably not at virialized states. We also find that most of the NH_3_ fragments are dynamically quiescent while only a few are active due to star formation activity.