The numerical model of the cross-correlation between pulses of the femtosecond laser sequence propagating in air is developed to investigate the temporal coherence characteristic of the femtosecond laser ultrashort pulse sequence. The model is based on the propagation theory of the femtosecond pulse sequence and the refractive index of air Ciddor formula. The auto-correlation patterns between pulses of the femtosecond pulse sequence in different spectral distribution with different optical path differences under different atmospheric conditions can be obtained from the model. The simulation results show that the auto-correlation patterns become broadening and chirp with the peak power decreasing due to dispersion, as the optical path difference between the pulses increases. However when the atmospheric conditions are changed with the same optical path difference, the correlations patterns only shift without any extra linear broadening or chirp. The femtosecond optical frequency comb has very high temporal coherence. The formation of the cross-correlation function between the femtosecond pulses only depends on the spectral distribution of the laser source.