In this paper, the scattering characteristics of microstrip antenna arrays in time domain are analyzed using finite-difference time domain (FDTD) method. The distributions of scattering field components, Eθ and Eφ, are studied when the polarization and incidence directions of the incident pulse vary. The relationship of time domain waveforms and spectra of the scattering field with the microstrip antenna array structure is analyzed. The effect of ground plane on the scattering field of microstrip patch array is investigated using the field separating approach. It was found that the scattering field of microstrip antenna array with finite ground plane is mainly generated by the edge current on the ground plane. The resonance frequency of microstrip patch array is associated with the polarization direction of the incident pulse, hence, different polarization directions correspond to different spectra of the scattering field.