The size and stability of the X-ray beam focal spot in the electron linear accelerator are crucial factors affecting the spatial resolution of high-energy industrial computed tomography. The commonly used “Sandwich” test method for the X-ray beam focal spot size measurement is complex, vulnerable to human-related factors, and impossible for real-time measurement of stability. In this article, we devise a slit translation scanning method to measure both size and stability of the X-ray beam focal spot for the electron linear accelerator. First, this article theoretically analyzed the impact of the effective slit width and sampling frequency on the measurement results. Second, a 9-MeV electron linear accelerator was used to measure the X-ray beam focal spot size. The measurement errors along x- and y-directions are smaller than 5.6% compared to the results measured by the IEC 62976–2017 standard method. Last, the stability of the X-ray beam focal spot was assessed using the slit translation scanning method with three different electron linear accelerators (9, 6, and 2 MeV). The results reveal random drift of the X-ray beam focal spot position, with a maximum drift of up to 0.6 mm. The experiments demonstrate the simplicity and efficiency of the proposed slit translation scanning method which proves invaluable for the meticulous measurement of the size and stability of the X-ray beam focal spot for electron linear accelerator.