Using a well-developed transient photocurrent model, we combine theoretical and numerical studies on the production of terahertz (THz) radiation in laser-induced air plasma. Air molecules are excited and then ionized by the laser-pulses mixing the fundamental and second harmonic fields. As a result, a non-vanishing directional photoelectron current emerges, radiating the THz pulse wave. We mainly discuss the influences of optical wavelength on THz radiation in five different cases. Results show that the fundamental-pulse width (50 fs), energy, and relative phase between two pulses are invariant. When the laser wavelength (from 400 nm to 1.25 \mu m) becomes longer, the emitted THz fields are significantly enhanced, with a linear dependence on the optical wavelength.