A long open-path tunable diode laser absorption spectroscopy (TDLAS) system based on wavelength modulation and second harmonic detection is developed. The method of normalized light intensity is adopted to eliminate the influences on the measurement by light intensity, and the measurement error is less than 2%. Then by using this system ambient methane is monitored at Dongpu island of Hefei with 750 m-optical-path. The measurement result is analyzed and the diurnal variation of atmospheric CH4 concentration has a very clear cycle in the Dongpu island. The general characteristics of diurnal variation are the lowest value in the daytime and the highest value at night, with the average concentration range of (1.8～3.4)×10-6, the highest concentration of (3.0～3.4)×10-6, and the lowest concentration of (1.8～2.2)×10-6. Experimental analysis shows that the system can achieve large region on-line monitoring of methane concentration with the distance of about 1～2 km and supply effective method to the monitoring of area emission flux of ecosystem and feasible ground calibration means to the satellite remote sensing.