Based on the Kirchhoff law, a practical temperature real-time measurement system using laser diode and LiTaO_3 Pyroelectric detector is presented. According to the curves of spectral radiance and relative temperature measurement sensitivity versus λT, working wavelength range of the system is given. By means of analyzing the relationships between sizes of cooled shelter plate and positioning method and working wavelength, and atmospheric infrared transmission, working wavelength of the system is determined. According to the effect on temperature measurement accuracy by sizes of cooled shelter plate and positioning method,H/R values are discussed. And at the same time, anti-jamming ability and temperature measurement inaccuracy are also analyzed. Five main reasons resulted in temperature measurement uncertainty are presented. And the two are briefly discussed. Temperature measurement uncertainty exceeds 0.3%.