The calibration- free measurement of gas parameters can be realized according to scanning- wavelength modulation spectroscopy technology which is based on tunable diode laser. A method based on an iteration algorithm which can be used to measure gas temperature, pressure and H2O concentration simultaneously is put forward. The iteration algorithm is less sensitive to initial values and has rapid convergence rate. Frequency division multiplexing technology is adopted to study the high temperature combustion flow using the harmonic signals of two H2O absorption transitions (7454.45 and 6 806.03 cm-1 repectively). Compared with thermocouple readings, pressure value of pressure transducer, and H2O concentration by direct absorption spectroscopy, experimental results show that the measured gas temperature, H2O concentration and temperature are consistent with the predicted value and the most relative errors are 7.6%, 8.1% and 7.5% respectively.