Abstract: For harmonic detection systems working in near-earth conditions, the harmonic radar equation derived under the free space hypothesis is not enough to predict and explain many phenomena in the measurement, so this paper studies the modified propagation model for such systems based on the characteristics of the target and environmental factors. The target harmonic characteristic is introduced in the propagation model, which makes the harmonic attenuation rate more in line with the actual measurement. Besides, the multi-path effect analysis of fundamental and harmonic signals allows the model to predict and explain the receiving power strength oscillation at a certain distance. In addition, simulations are conducted and measurement systems are set up. Indoor near-range and outdoor long-range experiments are designed to verify the predicted detection blind spot by the model. The proposed model has great significance for the design and performance evaluation of near-ground harmonic detection systems.