Abstract: The Xray source based on electron accelerator is the foundation of high energy industrial CT(ICT). A new type of small spot size SW electron linac was designed with high space resolution. This paper discusses that spot size of radial source influence on the imaging quality, and analyzes the physical reason that at high energy the small hole method wasn't suit for the measurement of spot size, it also gives four kinds of measurement method for this type of high energy Xray source spot size and the analysis of the experiment results. The measurement results show that spot size of the new type accelerator Xray source is about 1.4mm, and the limited space resolution is 2.5lp/mm.