Laser particle sizer which based on Mie scattering theory is one of the most widely used instruments in particle size measurement. Generally the smaller the particles are, the bigger the scattering angle is. The main peak position of Mie scattered energy distribution received by the detector array moves outward. However, for some particles of relative index of refraction in certain size ranges, the main peak position of scattered energy distribution moves toward the inside of the detector array as the particle size decreases, which can be called the abnormal moving of scattered energy distribution. Based on Mie scattering theory, the patterns of such abnormal moving and abnormal particle size interval of different relative indexes of refraction are obtained, the effect on particle size analysis is analysed, a method to reduce the effect is proposed. An actual sample is measured and compared. The results show that this method can reduce the effect of abnormal moving on particle size analysis.