Abstract: Toxic and harmful elements of living garbage can easily cause soil and groundwater contamination around the landfill. In-situ monitoring of harmful elements in landfills can provide the necessary technical support for safe and stable operation of landfills. In this paper, a method for detecting harmful elements in landfill based on instantaneous gamma-ray neutron activation analysis is proposed. The research model of the detection device for the actual environment of gas pipeline in landfill is made by using polyethylene ball, glass ball and ultra-pure water mixture as dry refuse substitute for preparing different landfill samples. The Monte Carlo method was used to simulate the geometrical parameters of the optimized model and experimental research platform was constructed using an americium beryllium neutron source with an activity of 300 mCi and a 4 inch BGO detector. Monte Carlo simulation and experimental results show that the characteristic peak intensities of chlorine, manganese, nickel and chromium in the landfill are enhanced with the increase of the element bulk density, and there exists a good linear relationship.