The experimental method by using the tunable diode laser absorption spectroscopy combined with the model and algorithm was studied to reconstruct the two-dimensional distribution of gas concentration. The feasibility of the reconstruction program was verified by numerical simulation. A diagnostic system consisting of 24 lasers was built for the measurement of H2O in the methane/air premixed flame. The two-dimensional distribution of H2O concentration in the flame was reconstructed, showing that the reconstruction results reflect the real two-dimensional distribution of H2O concentration in the flame. This diagnostic scheme provides a promising solution for combustion control.