Measurements of the fluctuations in solids flow in a circulating fluidized bed (CFB) boiler has been carried out with emphasis on the layer of downflowing particles at the furnace walls. Signals of local fluctuations in particle downflow, global pressure drop fluctuations in the upper and bottom zones of the furnace, and in absolute pressure in air plenum, were simultaneously sampled. Spectral and probability analyses on the signals suggest a regular slow fluctuation of the particle suspension flow passing through the core and falling at the wall. The flow of particles in the wall-layer has a wide distribution of densities. The transfer of particles from the core to the wall is the dominant process for the formation and evolution of the wall-layer in the furnace of CFB boilers.