When laser intensity varies, the ionization induced by optical field can be described by multiphoton ionization (MPI) in low regime and tunneling ionization (TI) in high regime. An empirical formula was used to fit the ionization happened between these two limitation ionization processes. Based on this, ionization rate and photocurrent induced by two-color femtosecond laser pulses interaction with air-plasma were investigated numerically. It was found that they have different relations with the laser intensity.