Abstract: Accurate evaluation of Global Navigation Satellite System (GNSS) interference effectiveness is a critical technical task for achieving navigation defense in key regions. Traditional GNSS interference typically relies on a single-station jamming transmitter to achieve short-range or small-scale interference on targets. However, with the advancement of anti-jamming technology, military GNSS receivers or navigation terminals are generally equipped with anti-jamming nulling antennas, rendering single-station interference ineffective. Furthermore, conventional GNSS interference effectiveness evaluation methods predominantly employ free-space propagation models to analyze signal attenuation during transmission, leading to significant discrepancies between theoretical analysis and practical engineering evaluation results. To address these issues, this paper proposes a distributed GNSS interference effectiveness evaluation method based on the irregular terrain model (ITM). By establishing a distributed interference coordination system, this method not only overcomes the anti-jamming mechanism of nulling antennas but also integrates the ITM radio propagation model to precisely calculate the physical effects (e.g., direct, reflection, diffraction, and scattering) caused by terrain, meteorology, and hydrology during actual signal transmission. Simulation experiments analyze the interference coverage (C_cov) and spatial variation characteristics under different terrain categories, station quantities, deployment schemes, and interference power conditions. The results provide a reference for strategy selection and application scenario design in practical navigation defense.