Selective catalytic reduction(SCR) is the most commonly used technology for reducing NOx emissions in coal-fired power generation. Ammonia gas(NH3) temperature in chemical reduction reaction is one of the key parameters for the closed-loop control of SCR process. A measurement method of ammonia gas temperature based on two-line tunable diode absorption spectroscopy is proposed. Real-time monitoring of chemical reaction temperature of SCR process was achieved. A pair of ammonia absorption lines(6605.1042 cm-1 and 6605.1901 cm-1) was selected according to HITRAN08 database. The relationship between the intensity ratio and temperature of two absorption lines was theoretically derived. The experimental results show that linear error and the maximum fluctuation of temperature measurement are 0.89%(average) and 3.5%(RMS) in 80~160 ℃ temperature range. It lays the foundation for the temperature and concentration measurement of ammonia under high temperature.