In this paper, based on TDLAS technology, an all-fiber NH3 concentration detection system was built by using the designed microfiber gas absorption cell. The core part of the NH3 detection system was sensed by a 1.51-m microfiber. The test results of the system indicate that there is a good linear relationship between the second harmonic amplitude and the corresponding concentration for NH3 in the concentration range of 20000 ppm-100000 ppm (correlation coefficient of fitting formula R=0.9962). To improve the detection performance of NH3 concentration, the gold-nanosphere (GNS) coated microfiber is used to enhance the evanescent field effect. According to the experimental results, the sensitivity of the microfiber coated GNSs NH3 concentration detection system has been greatly improved and the lower detection limit of NH3 concentration can reach 260 ppm. Repeated monitoring of different concentrations of NH3 shows that the detection system is stable with a maximum relative error of 5.38%, which makes it suitable for long-term stable NH3 monitoring and has wide application prospects.