Time-of-flight aerosol beams spectrometry is a typical method of accurate size and concentration measurement in aerosol particles' size distribution monitoring. Accurate measurement of aerosol particles' time-of-flight is the key to monitor the distribution of particles' size. A method of accurate measurement of aerosol particles' time-of-flight is designed by improving the threshold level comparison method, and the comparison of zero-level instead of the changing threshold level is realized based on differentiation since the maximum value of the original singal is corresponding to zero-level of the differential singal. By using this method, the time-of-flight can be accurately measured when the strength of the scattered light changes, or when the two peaks of the scattered light are not symmetrical.