The invention relates to a quantum cascade laser-based high-speed infrared frequency modulation laser spectrum gas detection system and method. The system comprises a quantum cascade laser spectrum scanning module, a high-speed frequency modulation module, a synchronization module, a gas multi-returning cavity and a frequency modulation spectrum acquisition module. On the basis of a conventional gas detection system, a distributed feedback quantum cascade laser is used, and a mid-infrared laser is utilized to generate intensity-modulated modulated light for irradiating and pulsating an emergent end face of the quantum cascade laser, thus realizing all-optical modulation on the quantum cascade laser; control and quantitative detection on a gas sample are completed by the gas multi-returning cavity; an all-optical modulation gas absorption spectrum is collected by a high-speed mid-infrared detector, a high-speed lock-in amplifier, a high-speed signal collection card and a computer, thus realizing high-speed high-accuracy gas detection. The high-speed infrared frequency modulation laser spectrum gas detection system can be applied to a high-sensitivity infrared laser spectrum technology and trace gas detection.