An improved finite difference method is used to study the field emitter arrays in both diode and triode modes. In the simulation, the boundary of the emitter can be fitted with irregular meshes. The emission performance of the emitter array is analyzed in this article. Simulation results show that the electric field at tips in the emitter array is not uniform. The emission currents from tips are also different. Due to the influence of the interaction and fringe field, the emission currents from tips near the edge of the array are larger than that from tips on the center part of the array. However, the presence of the gate can reduce the interaction between neighboring tips in the triode structure. Thus, it is possible to achieve a higher current density in the triode mode.