The intensity of photoacoustic signal is proportional to optical power. However, high power laser source has many disadvantages, such as high power consumption, complex driving and controlling circuit, the lasers with high quality and lower cost laser can't be attained easily. These kinds of laser source mostly focus on the band over 6 micron, which makes it difficult to detect the molecule in fundamental absorption band of 2～6 micron effectively. Moreover, photoacoustic gas sensors based on commercial driving and controlling instruments are more likely to have large volume, which can’t meet the needs of multi-points and continuous mobile monitoring. In this paper, nmol·mol-1 level measurement applied to molecule concentrations was realized in 3～4 micron band using low output power interband cascade laser (ICL) and a detection method which based on quartz tuning fork and photoacoustic spectroscopy. The ICL was used to target a strong ethane absorption ling at 2 996.88 cm-1 in its fundamental absorption band. High sensitive detection and system volume reduction were realized by using self-developed digital lock-in amplifier and digital laser driving and controlling method, as well as wavelength modulation spectroscopy technology. Moreover, data acquisition and processing process were simplified effectively. Firstly, system scheme and designing details of optical and electrical modules were introduced in sequence according to the system principle and structure. Simulations of target gas absorption and interference from other gases have been analyzed. Broadening and overlap conditions of absorption lines under different pressures have also been described. Working pressure of the system was determined to be 200 Torr finally. Secondly, the minimum detection limit (MDL) was deduced <100 nmol·mol-1 through performance test and analysis of one-circle spectrum scanning of ethane with 6 concentration levels (100～1 000 nmol·mol-1). The linear performance of this sensor was evaluated through ~10 min 2f peak value extraction test using aforementioned samples. The results indicated that the correlation was 0.999 65, and the relationship between gas concentration and the amplitude of 2f signal was clear. Finally, system noise was determined to be ~0.347 V by means of continuous 1 h test applied to nitrogen, thus the SNR and sensitivity were estimated to be ~28.56 and ~40 nmol·mol-1, respectively. The new mid-infrared C2H6 sensor introduced in this paper not only realized nmol·mol-1 level measurement but also greatly reduced the volume occupied by commercial instruments by use of digital driver and lock-in amplifier, which laid the foundation for realizing the goal of miniaturization and mobile measurement. In addition, for the applications with unlimited power consumption, sensor sensitivity can be further improved by using more powerful laser source or improving the performance of system modules such as lock-in and preamplifier, which can be applied in more fields.