This paper describes an infrared Imaging system for the Inner-formation Flying System (IFS), a new kind of spacecraft system to measure Earth's gravity field. Challenges arise when measuring the relative position between the moving inner-satellite and its cavity. The observing process should be real-time and highly precise, with the premise that non-gravitational disturbing force be restrained. An infrared imaging system was proposed for the observation. Challenges were handled via a thermal-electronic-mechanical integration design. With a specially designed electronic component, a dynamic thermal loop and a novel calibration mechanism, an infrared imaging system was established to perform long-time continuous precise measurement. Infrared images with a Signal Noise Ratio better than 2:1 was acquired, being sufficient to acquire accurate position of the inner-satellite. Performance of the imaging system was validated by experiment.