As an essential passive detection equipment in modern battlefields, the Infrared Search and Track (IRST) system’s imaging quality directly determines the accuracy of target detection and tracking. In dynamic environments, the relative motion between the target and the carrier causes image motion on the image plane, which not only degrades image quality but also poses difficulties for target tracking and acquisition. Effective suppression of image motion requires high-precision measurement as a prerequisite to achieve compensation control. To address this issue, a method for calculating dynamic image motion based on coordinate transformation is proposed. Firstly, an object-image conjugate model for the IRST system in dynamic environments is established, and the mapping relationship between object and image vectors is clarified through coordinate transformation to achieve accurate calculation of image motion displacement. Secondly, a six-degree-of-freedom motion platform experimental system is constructed to simulate various motion conditions under different carrier attitude disturbances and measure target image motion. Experimental results demonstrate that the theoretically calculated image motion values are highly consistent with the measured data, with a deviation of less than 2 pixels (RMS) and a relative error better than 0.67%. This method provides an effective technical approach for high-precision compensation of dynamic image motion and has important application value for improving the imaging stability of IRST systems in complex environments.