N2O is a significant atmospheric greenhouse gas that contributes to global warming and climate change. In this work, the high sensitivity detection of atmospheric N2O is achieved using wavelength modulation spectroscopy (WMS) with an inter-band cascade laser operating around 3.939 μm. A LabVIEW-based software signal generator and software lock-in amplifiers are designed to simplify the system. In order to eliminate the interference from water vapor, the detection was performed at a pressure of 0.1 atm (1 atm=1.01325×105 Pa) and a drying tube was added to the system. To improve the system performance for long term detection, a novel frequency locking method and 2 f/1 f calibration-free method were employed to lock the laser frequency and calibrate the power fluctuations, respectively. The Allan deviation analysis of the results indicates a detection limit of～20 ppb (1 ppb=1.81205 μg/m3) for a 1 s integration time, and the optimal detection limit is～5 ppb for a 40-s integration time.