Spectral analysis on the transmission fluctuations is a new method for measurements on large particles. Radiated by a narrow beam, the flowing particle system at a uniform velocity causes the transmission signal fluctuate violently. Both the average transmission and the spectrum of transmission fluctuations can be obtained statistically, which include the complete information on particle size distribution and particle concentration (volume fraction). By using the inverse algorithm, the particle size distribution may be extracted from the spectrum of transmission fluctuations and consequently the particle concentration can be achieved from the average transmission. The theoretical expression of the transmission fluctuation spectrum is established, for monolayer of mono-dispersed particles. The theory is then extended to a 3-dimensional system of mono-dispersed particles and further to that of poly-dispersed particles. Experimental test and simulation on dilute particle systems with particle diameters in the range 32～425 μm are performed, which shows that the method is valid to measure the particle size distribution and particle concentration simultaneously.