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 nonuniformity of temperature and water vapor concentration. The effect of the number of temperature bins on column-density and temperature results has also been explored.