Contaminants on the optical surface is one of the key factors that reduce imaging quality of optical imaging system. The imaging system,which is contaminated by particles with the diameter d=0.3 mm and different distances between particles,is examined firstly. The relevant resolution power is obtained by software HYRes 3.1,which is written by Olympus Corporation. Then FDTD (finite-difference time domain) method is used for calculating the distribution of forward scattering intensity. The results show that the maximum intensity changes a little,but location is away from center of image plane,and light energy concentrates on some point as distance between particles falls. At same time,resolution declines. When distance between particles reaches a fixed value,resolution reduces so badly that it cannot be read out by software,and maximum light intensity,which is situated at edge of image plane,drops rapidly. Therefore,the change of the light-energy distribution brought by scattering of particles can reduce the imaging quality of optical imaging system when contaminants quantity on optical surface is too small to change transmissibility.