Based on the variational principle of nonlinear Schr?dinger(NLS) equation describing laser pulses propagation in air, the properties of femtosecond filamentation were studied by using numerical simulation method. Numerical results were derived on the developing process of filamentation under the situations of whether or not taking account of multiphoton absorption dissipation (MPA). It is shown that a dynamic equilibrium between optical self-focusing and plasma defocusing will be broken due to the MPA, which can induce instability of femtosecond filamentation. The first foci of the filaments are displaced for different incidence laser power. Further investigation reveals that stabe filamentation propagation described by defocusing-refocusing cycles can only arise for power pin?3pcr in laboratory. With the incidence power gradually increasing, the beam will undergo optical collapse due to intense focusing, and then the formation of femtosecond multiple filamentation(MF) can be obtained.