Abstract: Characteristics of ion noise in a traveling wave tube (TWT) focused by periodic permanent magnets (PPM) have been discussed by means of a hybrid model. One dimension Particle-in-Cell method (PIC) simulation code is developed. Compare to uniform magnetic field focus system the noise frequency is lower and the spectrum is wider by the PPM focus system. From the analysis results we find decreasing the axial electrostatic potential well would reduce this noise.