Ammonia is one of the main odors. A mid-infrared laser based gas sensor was developed for continuous monitoring of the ammonia in pollution emissions in the industrial environment. Different from the traditionalnear-infrared ammonia sensor, a mid-infrared distributed feedback inter-band cascade laser (DFB-ICL) was used as light source, tuned at the center of 3 μm, which could qualify the sensor to avoid from the interference of water vapor as well as carbon dioxide. A hollow silica waveguide was used for gas cell, and the core diameter is 1 mm, the length is 5 m. A home-made multi-channel digital lock-in amplifier was adopted to demodulate 1f/2f harmonics simultaneously so as to realize calibration-free measurement. Concentration gradient results show that this sensor has a linearity as high as 0.99917 as well as an uncertainty of 0.9%. Based on the Allan variance method, it can be clearly seen that the sensor shows a quite excellent stability performance with a detection limit as low as 9.7 ppb at the optimum integration time of 167 s.