The registration accuracy of image fusion is an important performance index that is related to the quality of image fusion. The infrared and visible image fusion objective optical system in this study adopts a parallel optical path layout and an optical passive thermal compensation method to improve the registration accuracy of image fusion. First, the contributions of mechanical thermal compensation and optical thermal compensation are analyzed and compared to improve the image registration accuracy. Second, according to the performance index of the image fusion objective system, the optical passive thermal compensation designs of the infrared objective and visible objective are optimized. Moreover, the necessity of optical passive thermal compensation design of the visible objective is analyzed. Finally, according to the imaging quality and image registration effect of the image fusion objective system, it is concluded that the quality of the fused image is high and that the requirements of the index can be achieved.