Abstract: The minimum focal spot to skin distance is a critical parameter in the national standard to restrict the miniaturization of CT scanners. This paper introduces the physical meaning of focal spot to skin distance, analyzesthe similarities and differences between X-ray fluoroscopy imaging and X-ray computed tomography, and investigates the important effect of the filter on the absorbed dose distribution in the scanning hole in X-ray computed tomography. The experimental results show that, due to the effect of the filter, the absorbed dose distribution in the CT scanning holes of the large and small CT scanners is high in the central region and low in the inner wall region. The concept of minimum focal spot to skin distance from X-ray fluoroscopy imaging does not perform the same function in computed tomography, but it limits the development of miniaturization of CT scanners.