We investigate the terahertz (THz) wave emission from air plasma by analyses and simulations. An elliptically polarized THz wave is generated, whereas a circularly polarized carrier-envelope phase (CEP) stabilized few-cycle laser pulse is applied. Its ellipticity and intensity depend on the pulse duration of the driving laser pulse. And the polarization rotates along the CEP of the driving laser pulse. The THz generation is also simulated for different filament lengths. As the filament extends, the polarization of the generated THz wave rotates along the filament.