With the progress of the laser manufacturing technology, trace gas sensors based on tunable interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) have been widely used to detect organic compounds with high sensitivity. Compared with overtone and combination bands in the near infrared region, for many species, the intensities of fundamental rotational-vibrational absorption bands in the mid-infrared region are much stronger. In this paper, we demonstrate an ethanol sensor using a room-temperature continuous-wave (CW) tunable ICL laser as a light source to detect ethanol vapor concentration with high sensitivity. Combined with the first harmonic (1f) normalized second harmonic (2f) wavelength modulation spectroscopy (WMS) technology, the characteristics of the harmonics of the system are analyzed, and the amplitude of the first harmonic decrease with an increased concentration of ethanol has been demonstrated both theoretically and experimentally. As a result, a detection limitation of 28 ppb is achieved.