Based on neutron moderation theory, a nondestructive method is proposed for quantitatively detecting millimeter-scale voids at steel shell-concrete interfaces under complex structural conditions. First, an indoor calibration experiment is conducted to establish the relationship between thermal neutron counting rates and void heights, using steel plates with thicknesses of 14 and 40 mm, as well as three concrete blocks of varying moisture contents. The experiment clarifies the relationship among steel plate thickness, concrete moisture content, void height, and thermal neutron counting rates in steel shell-concrete composite structures. Next, a verification test is performed that utilizes a theoretical curve for a 40-mm steel plate and a concrete block with specific moisture content. The results show that the equivalent void height measured through neutron scattering exhibits a relatively small error. Finally, a case study is conducted on the steel shell-concrete immersed tunnel of the Shenzhen-Zhongshan link, where a full-scale cross-sectional model is established for on-site void detection and open-cover validation. Following removal of the steel plate cover, void heights at the detection points are measured via the overflow method and compared with the detection results. Statistically, with an allowable error of ±2 mm, the compliance rate for void height measurements reaches 94%.