Based on the theoretical model of field ionization current for terahertz (THz) emission in laser-gas interaction, the parameter optimization for strong THz emission via the two-color laser scheme is explored. It is found from numerical calculation that the ionization current is due mainly to the first-order ionization process and the contribution from high-order ionization is ignorable. In order to produce stronger THz emission, the ionization current can be enhanced by adjusting the amplitude ratio between the fundamental and its second harmonic laser pulses as well as their relative phase difference. The ionization current can also be increased by use of laser pulses at mid-infrared and by changing the gas species. Under the same laser pulse and gas density conditions, the ionization current from the Helium gas is almost twice that from the Neon gas.