The effect of the free mode on the trajectory of the solutions to a forced dissipative system is numerically researched.First,the free mode is obtained from observed atmosphere and a type of analytical free mode(ideal free mode)is designed;Secondly,using two kinds of the free modes as forcing fields respectively,and the 500hPa streamfunction field for July 1,1993 as the initial field, then the numerical experiments are run for 300 days by a forcing and dissipative global spectral model.Results indicate that when using the observed free mode for December 27-28,1992 as the forcing field,and α=γ=κ≥0.02,that is to say,when the forcing and dissipative coefficient is equivalent and great enough,the model atmosphere always approaches to the free mode as time evolves;when the forcing coefficient α=0.1 and the dissipative coefficient γ=0.02,the local quasi-biweekly oscillation appears in the extratropical latitude.When using the free mode of idealized atmosphere,dissipative coefficient 0.01,and forcing coefficient 0.0095≤α≤0.011, the period of oscillation has a positive correlation with the amplitude of the forcing coefficient.