Starting from temperature-measuring principle of three primary-colors, certain factors in the application of measuring flame temperature field was discussed. A convenient temperature-measuring formula was deduced and method to rectify measuring errors by calibration of system and relationship of three colorimetric temperatures was put forward. Through a designed measuring system, it measured temperature field of 200 MW boiler, and compared it with the measured data of dual spectrum thermometer. Results showed that the way described above is simple, the error s is no more than five percent, and has high measuring accuracy to satisfy the purpose of systematic burning diagnosis and real-time measurement as well as control, resulting in certain applied prospects in engineering.