The low-altitude security warning and defense system is a low-altitude aircraft warning and defense system under complex urban environment developed to address the social hot topics about low-altitude security in the current high value-added important economic goals (such as party, government, military, water, electricity, gas, oil, chemicals, and transportation), while avoiding the impact on the surrounding electromagnetic environment and residential environment. It enables the detection and recognition of low-altitude aircraft signals, acquisition of identity information, acquisition of communication content, vulnerability discovery and utilization, and decoy landing by same frequency multi-target separation, adjacent amplitude direction finding, multi-domain fusion low-altitude aircraft detection, and protocol cracking. It allows the full-automatic systematic work from warning, monitoring, identification, tracking, differential strikes, and fixed-point forced landing, and has such advantages as low cost, flexibility, and easy operation. This system reserves a hard anti-personnel weapon interface; can be applied to border port spectrum monitoring and military operations; meets the current needs of the battlefields and markets, and fills the gap in the field of low-altitude security of high-value-added targets in civil and military defense. It is movable, vehicle-mounted, fixed, stand-alone, and networked.