Abstract: The uranium neutron coincidence collars (UNCL), as a safeguard device based on the neutron coincidence counting method (NMC), is often used to measure the ²³⁵U linear density of fuel assemblies. It is of practical significance to clarify the sensitivity of UNCL devices to the replacement of fuel assemblies for expanding their application in the field of nuclear non-proliferation. Therefore, based on the Geant4 toolkit, the UNCL device model was constructed, and the impact of fuel assembly replacement detection and the influence on gadolinium-containing fuel assemblies were studied using the device model. The research results show that the device has a relatively low sensitivity to the fuel rods far from the neutron source and the polyethylene plate. By replacing the ²³⁵U enrichment of the fuel rods in this area and comparing the relative deviation of the neutron coincidence count rate before and after the replacement, it can be found that, for highly enriched fuel rods, the relative deviation before and after replacement is 6.93% to 28.74%. In contrast, the relative deviation of the fuel rods with a lower ²³⁵U enrichment degree was only −1.09%. In addition, by replacing the standard fuel rods in the fuel assembly with gadolinium rods of different contents and quantities and conducting compliance measurements, it can be found that, the device is extremely sensitive to the presence of gadolinium rods in the fuel assembly. The replacement of gadolinium-containing fuel rods can lead to a relative deviation of the neutron coincidence count rate as high as −11.85%. In the case where there is a replacement of fuel rods with high enrichment in non-sensitive areas, the presence of a certain number of gadolinium rods will make it difficult for the device to detect the replacement.