According to the fundamental absorption properties of ethane (C2H6) near 3.3 μm, a mid-infrared C2H6 sensor based on a wavelength modulation spectroscopy (WMS) technique was developed using a room temperature, continuous-wave (CW) interband cascade laser (ICL) emitting at 3.34 μm and a dense multi-pass gas cell (600 mL) with a 54.6 m optical path length. The principle of gas detection using spectral absorption method based on wavelength modulation spectroscopy and two harmonic (2f) detection technology is introduced in detail. Selection details of the target ethane absorption line are also given. The use of this technology reduces the influence of optical power drift on the system, making the minimum detection limit (MDL) and stability performance of the system get promoted. Ethane sensing system is introduced in detail through optical and electrical modules combined with the scheme. The application of self-developed software and hardware units as well as commercial instruments and their model are described for the reference to others, and physical map of the sensor optical core is also given. Moreover, the pressure and modulation depth are optimized in order to match the wavelength modulation of laser and absorption linewidth based on gas pressure. The curves of the modulation amplitude corresponding to the peak value of 2f signals and the modulation depth corresponding to modulation depth are also drawn accordingly, and finally the appropriate pressure and modulation depth are determined to be 100 Torr and 0.074 cm-1, respectively. The corresponding modulation amplitude is ~0.026 V at that point. In addition, the work of system sensitivity estimation is conducted by using 136.8 nmol·mol-1 C2H6 standard gas based on the optimized air pressure and modulation depth. The parameters setting of ICL scanning and modulation signals, phase-locked amplification as well as data acquisition are introduced in details, and pictures recorded by oscilloscope are also given. In this case, the system MDL is estimated to 33 nmol·mol-1 by comparing 2f signal acquired by DAQ and background noise signal. Finally, the fitting curves and its correlation information are described by carrying out ~5 minute system calibration tests, respectively, by using 9 different C2H6 standard gases from 20～400 nmol·mol-1. Moreover, 2 hours system stability test was conducted by using 48 nmol·mol-1 C2H6 sample. The result shows that this system works steadily and a minimum detection limit (MDL) of ~0.81 nmol·mol-1 is achieved with a measurement time of 4 s. The MDL is further improved to 0.36 nmol·mol-1 with a measurement time of 63s, based on an Allan deviation analysis for the C2H6 sensor operation.