In order to accurately measure volume fraction of the escaping ammonia at high temperature, tunable diode laser absorption spectroscopy, wavelength modulation spectroscopy (WMS) and long-path technology were used. A set of miniaturized measurement instrument for escaped ammonia gas at high temperature was developed. In order to reduce the adsorption effect of escaping ammonia and improve the detection sensitivity, a new type of sample absorption cell with high temperature and long optical path was developed. On the basis of the laser driving module developed earlier, phase-locked loop chip 74HC4046 was used as an adjustable sinusoidal modulation signal source. EPM7064 was used as phase shifting and frequency doubling logic control chip. At the same time, two AD630 chips were used as simultaneous demodulation multiplier of the first demodulation (WMS-1f) and the second demodulation (WMS-2f). The synchronous demodulation of 1f and 2f absorption signals was realized. In addition, STM32F429 was used as main controller and the demodulated and filtered signal was input to AD7606 for analog-to-digital conversion. Digital filtering and volume fraction inversion were performed. The results show that linear fitting coefficients of volume fraction of ammonia gas with WMS-1f amplitude, WMS-2f amplitude and the normalized amplitude of WMS-2f/WMS-1f are 0.998, 0.997 and 0.998, respectively. Allen variance shows that when the optimization time is 228s, the lowest volume fraction is 0.496×10-6. In the range of volume fraction 20×10-6~100×10-6, the measurement error is less than ±2%. The instrument can provide high precision raw data for high temperature measurement of ammonia escaping from coal-fired power plants.