A method and application of measuring the soot volume fraction, H2O concentration, and its temperature simultaneously in an ethylene/ air premixed flame environment based on tunable diode laser absorption spectroscopy technique is presented in this article. Only one distributed feedback diode laser with a center wavelength at 1397.83 nm combined with a scanned-wavelength direct absorption spectroscopy technique was used during measurements. The attenuation of laser intensity caused by soot extinction and H2O gas absorption after transmitting through the flame region was detected and distinguished, respectively. Soot volume faction was inferred from the light extinction with Rayleigh limit assumption. The absorption signals of two H2O spectral lines were acquired during one scanning, thus both the H2O concentration and its temperature can be retrieved. Measurements have been taken in ethylene/air premixed flat flames on a standard bronze McKenna burner at different cold-gas velocities (5.9 cm/s and 4.4 cm/s) and equivalence ratios (phi = 2.3, 2.1, and 1.9). The gradient tendencies of soot volume fraction, H2O concentration, and temperature along the vertical direction and the variations in different flames were well demonstrated. The results were in good agreement with the values measured using other techniques and studies. The proposed method is proved to be suitable and accurate for measuring oot volume fraction together with gas concentration and temperature. This multipurpose measuring method with high accuracy exhibits great potential in combustion diagnostics and soot study.