In the context of global terrorism and violent crime, concealed firearms pose a serious threat to public safety. However, traditional security screening methods suffer from shortcomings such as ineffective non-metal detection, ionizing radiation risks, and low efficiency. Active terahertz imaging technology, with its non-ionizing radiation, strong penetration capability, and high resolution, has emerged as an effective solution for concealed firearm detection. This paper reviews the current status of active terahertz imaging technology in firearm detection. Firstly, it elaborates on the physical characteristics of terahertz waves and the principles of active terahertz imaging, analyzes the technical features of key modules including transmitters, scanning systems, and core algorithms, and categorizes terahertz radar imaging modalities. Secondly, it summarizes research progress in active terahertz imaging for firearm detection both domestically and internationally. Finally, it analyzes technical bottlenecks such as high equipment costs, high-humidity attenuation, undersampling-induced image blurring, insufficient real-time performance in complex environments, and limited portability, while outlining future development directions including multimodal fusion, intelligent algorithm optimization, and standardized system construction. In conclusion, active terahertz imaging has enabled efficient detection of concealed firearms in airports, subways, and other scenarios; future breakthroughs in commercialization will require domestic production of core components and algorithmic innovation.