Measurement of combustion temperature is critical to understand combustion processes, improve combustion efficiency and reduce the production of pollutants. A single diode laser sensor system is established to obtain on-line measurement results of combustion temperature. The measurements are performed in a flat burner based on scanned-wavelength direct absorption spectroscopy by using two adjacent water lines 1397.75 nm and 1397.87 nm. The real-time results are acquired by using a DAQ card with Labview data processing program and the standard uncertainties of temperature are 2.3% at the temperature of 1750 K. The boundary layer of flame impact on the line-of-sight results is analyzed. It is demonstrated that the line-of-sight results is not the path average value, and may affect easily from the nonuniformity of the combustion.