The transmission fluctuation spectrometry (TFS) is a new method based on the spectral characteristics of the transmission fluctuation spectra, which is developed recently for real-time, online/inline particle analysis in two-phase flow. Both the particle size distribution and concentration can be measured simultaneously. The characteristics of the transmission fluctuation spectra with the 1 st order band-pass filters is presented detailedly. The effects of the filter band width parameter on the spectrum and on the inversion results are analyzed together with the effects of the dimensionless beam diameter (beam-to-particle diameter ratio). Numerical calculation and experiments show that a small value of band width parameter can improve the resolution of measurements but reduce the strength of the spectrum and hence bring errors into the results. On the contrary, a big value of band width parameter is beneficial to obtain a strong spectrum but lead to low resolution of measurement.