Large zooming ratio thermal imager based on medium-wave(MW) 640 pixel×512 pixel(15 ?滋m) MCT staring focal plane detector has been achieved. The thermal imager has wide field of 35°×28°capacitating wide area surveillance by 35× continuous zoom optical lens and the instantaneous field of view (IFOV) is 0.027 mrad/pixel. In the standard atmospheric conditions, this thermal imager can detect 4 m× 3 m×2.3 m vehicles target at 55 km and can identify the same target at 15 km (identification probability of 50%). This high performance has met the needs of the modern long-distance photoelectric weapons system. There are four key techniques during the design of the high performance imager including zoom optical system design of smooth change roots compensation curve, the single rail/double slider zoom structure technology, adaptive servo control technology and infrared image enhancement technology. The image is always clear and has no breakpoint in the continuous zoom process of the whole large variable ratio. The excellent Minimum Resolvable Temperature Difference (MRTD) of this imager has been achieved at Nyquist frequency(18 cyc/mrad). Performance data and imager photos have been presented. Test results show that the thermal imager has good performance and the above four key techniques of the thermal imager have been achieved from the theory design to the machine system engineering research.