There is a strong non-linearity between the field emission current amplitude and field intensity. This nonlinearity degrades the dynamic range of digital signal processing in a mechanical oscillator composed of a cantilever. To solve this issue, we propose a linearization method by combining the harmonics of the field-emission current. The proposed method significantly increases the dynamic range. The nonlinearity error is 0.00894 %FS for the proposed method, whereas it is 14.1%FS for the conventional method given by the 2nd harmonic current detection.