The THz generation efficiency and the plasma density generated by a filament in air have been found anti-correlated when pumped by $800\text{nm}+1600\text{nm}$ two-color laser field. The plasma density near zero delay of two laser pulses has a minimum value, which is opposite to the trend of THz generation efficiency and contradicts common sense. The lower plasma density cannot be explained by the static tunneling model according to the conventional photocurrent model, but it might be attributed to the electron trapping by the excited states of nitrogen molecule. The present work also clarifies the dominant role of the drifting velocity accelerated by the two-color laser field during the THz pulse generation process. The results promote our understanding on the optimization of the THz generation efficiency by the two-color laser filamentation.