A novel technique for characterizing temperature non-uniformity has been investigated based on measurements of line-of-sight tunable diode laser absorption spectroscopy. It utilized two fiber-coupled distributed feedback diode lasers at wavelengths around 1339 and 1392 nm as light sources to probe the field at multiple absorptions lines of water vapor and applied a temperature binning strategy combined with Gauss–Seidel iteration method to explore the temperature non-uniformity of the field in one dimension. The technique has been applied to a McKenna burner, which produced a flat premixed laminar CH4–air flame. The flame and its adjacent area formed an atmospheric field with significant non-uniformity of temperature and water vapor concentration. The effect of the number of temperature bins on column-density and temperature results has also been explored.