Soil nailing has become a common method for improving slope stability due to its cost-effectiveness and easy installation. However, concerns have been arisen regarding the quality and integrity of soil nailing works. This study proposed a novel method for soil nail integrity assessment using an actively heated fiber-optic (AHFO) sensor. First, numerical simulations were conducted to determine the thermal conductivity profiles of soil nails with various defects, measured using AHFO. By establishing the relationship between defects and thermal conductivity profiles, the material and severity of soil nail defects can be estimated. It was found that the thermal conductivity profiles obtained by the AHFO sensor accurately correspond to defect locations, and the entrained material and severity of defects in the section can be identified for defects with a length exceeding 0.1 m. Physical model tests were performed to further validate the feasibility of the proposed quantitative soil nail integrity testing method. This paper also provides recommendations for heating parameters and testing processes to facilitate the implementation of this method by potential users.