Transmission fluctuation spectrometry 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.The transmission fluctuation signal is averaged within a variable time interval in the time domain and is then expressed in terms of the expectancy of transmission square.By changing the time constant,the transmission fluctuation spectrum is obtained,which includes the information of particle size distribution and particle concentration.In this work,the theoretical expression is deduced,which also takes account of the influence from the particle concentration.