Laser-induced breakdown spectroscopy (LIBS) is one of the effective ways to achieve online monitoring of solid material characteristics by detecting the particle flow directly. We investigated the different particle sizes of high purity quartz sand, whose stable flow was produced by high precision continuous powder feeding machine. 5 groups of quartz sand flow samples with different particle sizes were then ablated by a pulse laser directly to test the particle size effect and its modified method. The results show that slight fluctuation of mass flow has no obvious effect on the spectral intensity, while the fluctuation of particle size has significant impact on the spectral intensity. The spectral intensity increases with the decrease of particle size. By analyzing the time characteristics of Si spectral line intensity of each particle size, it is found that the effect of particle size is weak during the 2-4 μs delay time. Besides, the directly, synchronously excited and generated N spectral lines were used as an internal standard of silica spectrum, which may further weaken the effect of particle size, thereby obtaining the corrected spectral intensity which can characterize the particle flow. In this experiment, the correction effect of 343.76 nm line is best.