A continuous zoom infrared imaging system with a large zoom ratio can be used for target searching in a large field of view and for high precision tracking in a small field of view at the same time. Because of the large zoom ratio, to keep good imaging quality throughout the zoom range becomes the difficulty for design and validation. To solve this problem, the Modulation Transfer Function (MTF) is used to measure and validate the variation of the zoom image plane. There are two main methods for measuring the MTF of an infrared imaging system: a slit method and a sloping edge method. After analysis and comparison of the characteristics and influence factors of both methods, the sloping edge method is chosen. By using the MTF to calibrate the defocus curve of the existing test sample with a large zoom ratio at each focal length, the optimal image plane position is obtained. Compared with the theoretical design value, its maximum deviation is 36 μm. The focusing compensation is modified according to the data. Finally, the maximum deviation is reduced to 5 μm.