The present invention discloses an infrared continuous zooming optical system with a large relative aperture and a high zoom ratio. The system comprises in order from an object surface to an image surface: a front fixed lens set, a first zooming lens set, a compensation lens set, a second zooming lens set and a rear fixed lens set. In the zooming process, the first zooming lens set, the compensation lens set and the second zooming lens set are continuously arranged in order, and a high zoom ratio exceeding 50 times and a relative aperture with F being constantly 2 can be obtained at the same time in a mode for separated motion front and back along an optical axis. The zooming optical type is concise, the zooming track is smooth, fixed components and motion components can be separately installed, regulated and located for easy implementation of an optical machine engineering. The infrared continuous zooming optical system can perform continuous zooming, is large in zoom ratio, large inrelative aperture, few in the number of lenses and high in energy collection, is especially suitable for a zooming system with medium waves of 3-5[Mu]m and infrared bands of 8-12 [Mu]m and can achieve100% cold light aperture matching.