Abstract: In order to reveal the response relationship between the neutrons and void height of steel-shell concrete composite structures, and establish a new quantitative detection method for the void height, based on the principle of fast neutron moderation and neutron scattering, an indoor equivalent concrete physical simulation method was used to construct a void model and conduct tests in this paper. Univariate regression analysis was carried out on the tests data, and it was found that there was a significant linear correlation between the thermal neutron count rate and the steel plate thickness, concrete moisture content and void height in the steel-shell concrete composite structure. The calibration relationship between the void height of steel-shell concrete and the thermal neutron count rate was further established, and the verification test was carried out. The results show that the error between the concrete void height value calculated by the calibration relationship and the actual void height value is small, the two have good consistency, the thermal neutrons generated by the action of fast neutrons penetrated through the steel plate and concrete can be used to quantitatively calculate the void height of the steel-shell concrete composite structure, the neutron method can solve the problem of inaccurate quantification in current detection.