The discrete element method and statistical analysis of contact networks were combined to study the packing structure and it's crystallization for disk systems under isotropic compression. In the discrete element simulation, the dispersion degree was controlled by changing the particle size ratio of large and small particles to make the system become monodisperse or approximate monodisperse. The percentage of contact types was used to describe the statistical characteristics of contact structure. The curves of the pressure of the system and the percentage of various contact types varying with the packing fraction were given, the influence of friction and particle size ratio on these curves was studied, and the effects of friction and monodisperse degree on the crystal lattice structure were emphatically discussed. The simulation results show that with the increase of the number of disks, the critical packing fraction increases gradually, with the increase of friction and particle size ratio, the critical packing fraction decreases gradually. Friction and the level of monodispersity are the main factors affecting the extent of crystallization of contact structure. In the monodisperse disk system, when the friction coefficient is small, triangular lattice clusters are easily formed in the continuous compression process of the system, with the increase of friction coefficient, the extent of crystallization decreases. When the friction coefficient is greater than 0.8, it is difficult to form triangular lattice clusters. In the approximate monodisperse disk system, only when the level of monodispersity is very high, the continuous compression process will lead to obvious crystallization. With the increase of particle radius ratio, the crystal lattice structure is more difficult to appear, and the system usually forms disordered packing.