For the purpose of enhancing the stability and enlarging the dynamic range in trace gas sensing, the conventional wavelength modulation spectroscopy technology was improved by the introduction of logarithmic-transformed data processing method and differential detection circuit. Before the extraction of the gas absorption related harmonics using a lock-in amplifier, the logarithmic-transformation and the differential detection were fulfilled by a homemade receiver. Through the logarithmic-transformation, the optical intensity modulation of the laser emitting was separated from the absorption-induced power attenuation, and then the former was balanced during differential detection. Owing to this two-pronged strategy, each harmonic component of the absorption spectrum can be theoretically captured without the interferences of residual amplitude modulation and harmonic distortion. For the validation of theory, the second harmonic of P(6) absorption line for NH3 was acquired. The experimental temperature and total pressure were maintained at 296 K and 1.01×105 Pa, respectively. Under the effective path length of 24.5 cm, a 0.7 ppm (part per million) detection limit was deduced on the assumption that the amplitude of signal was weakened to be equal with the noise. The above results indicate that this scheme is an ideal option for trace gas detection application.