Abstract: Large Aperture Scintillometer was widely used to measure horizontal distance (250~4 500 m) surface energy balance and fluxes of water and heat. In transmitting infrared waves, several environmental factors and land surface properties affect LAS signal. Further given the complexity of external conditions, high trees and electromagnetic fields of high voltage wires in the path of LAS light also blur receiver signal detection. Through experimentation at Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences in August 2009 through March 2010, this paper, analyzed the effects of electromagnetic fields of high voltage wires on LAS observation. The result indicated when the space between the path of LAS infrared waves and high voltage wire was too narrow, measured sensitive heat flux (H_LAS) was greatly affected, causing the difference between the calculations and the practice. Following detailed analysis of the experimental data, the paper suggested safe distance, in which electromagnetic field intensity should be less than 4.05 μT, from high voltage wires during LAS installation in densely populated areas. The study would extremely benefit and improve future LAS observations.