In this paper a new mechanism to form the proton density distribution to explain the radial evolution of the relative beam density (nb/np) is suggested. It is found that it is the resonance between protons and cyclotron waves that drives the protons to form such a unique distribution. First, several possible cyclotron waves are calculated to give the cold plasma dispersion relation. Among those possible waves, the waves propagating outside will play an essential role in the beam distribution formation and t...