Surface-deposited particles can impact on the optical performance of space optical communication. A model of antenna surface contamination is set up by finite-difference time-domain(FDTD) method. Transformation from near field to far field is made, basing on the near-field light intensity distribution calculated by FDTD for emission and receiving are all from far field. It is remarkable that distribution of near-field transmitted light intensity reaches a certain value, when distance between certain radius particles reaches a value, and the distribution is regarded as a reference. The relevant maximum distance between particles is obtained, with the same particles size and different wavelength. The results show that for a certain distribution of light intensity, the longer the laser wavelength is, the greater the relevant distance between particles is on the condition that the particle size is much larger than the wavelength. While designing the defence of space optical system, the big particles forming on the optical surface should be reduced for longer carrier wavelength.