Abstract: Pumped by two-color field, generated THz field amplitude and polarity depend on relative phase between two frequencies. Due to dispersion in plasma filament and Gouy phase shift, relative phase of two-color field varies along laser propagation direction. As the phase changed by , local THz field in filament got polarity reversal, far field THz constructive enhanced in off-axis direction, while destructive interference in forward direction. By adjusting the focusing geometry for two frequencies and through nonlinear dispersion in plasma filament the relative phase of two-color field was modulated. It was found that the relative phase flattens by the plasma formation and the conditions of focusing geometry. The dephasing length of THz fields that maintaining its polarity was elongated, with maximum value three times larger than dephasing length in ambient air. The far field THz radiation was enhanced in forward direction near the propagation axis.