In this paper the evolvement of the filamentation of femtosecond laser pulses propagating in air is studied experimentally.We observed the variation of light intensity in the beam profile and the interaction of the filament with the distribution of light intensity in the forming of the plasma filament by using imaging.The experimental results show that the laser pulse propagating in air first forms two filaments and then merges into a single filament.The numerical simulation using the nonlinear Schrdinger equation coupled to a multiphoton ionization law appears to describe several experimental results quite well.