In order to realize the online measurement of hydrocarbon fuel ramjet engine plume temperature, the spectral line pair of (7 444. 352+7 444. 371) cm -1 ~ (7 185. 586 5+7 185. 597 3) cm -1 was optimally selected through molecular spectral simulation taking H2O molecular as the target composition, and tunable diode laser absorption spectroscopy (TDLAS) system was designed to measure the parameters of hydrocarbon fuel ramjet engine plume using scanned-wavelength direct absorption spectroscopy-time division multiplexing (SDAS-TDM) strategy. Moreover, the temperature measurement accuracy of this system was verified by premixed flat-flamed burner. The results show that the relative deviations between the measured flame temperature at the height of 1cm of the premixed flat-flamed burner using this system and the standard reference value are all within 15% . On this basis, the system was applied to measure the parameters of the hydrocarbon fuel ramjet engine plume. Through alternately modulating of 1 392 nm and 1 343 nm diode lasers, the intensity attenuation signal of the modulated laser passing through the measuring zone was measured, the H2O molecule absorption spectrums at (7 444. 352 + 7 444. 371) cm -1 and ( 7 185. 586 5 + 7 185. 597 3) cm -1 lines were obtained. The online temperature measurement of the hydrocarbon fuel ramjet engine plume was realized with two-line technique, which provides important references for the combustion organization and performance evaluation of hydrocarbon fuel ramjet engine.