Abstract: Abstract The Time-Domain Electromagnetic Reflection (TDR) method is widely used in nondestructive testing due to its non-invasive and high sensitivity and it has now been applied in the measurement of soil nail lengths. Firstly, the basic principle of the TDR method was analyzed. Then, numerical simulations were conducted for different scenarios including variations in soil nail radius, distance to parallel conductors, grouting materials, end connectivity conditions and excited main frequency. The propagation characteristics of electromagnetic waves between concrete and soil nail were analyzed,and the factors affecting the length of soil nails using time domain electromagnetic reflectivity method were determined. Finally, experiment on soil nail length measurement were conducted, the detection results of different soil nail lengths and intermediate medias were compared. The results indicated that the material of the grouting was a significant factor affecting the accuracy of soil nail length measurement using the TDR method, while the soil nail radius, distance to parallel conductors, end connectivity conditions and excited main frequency had no effect.