The measurement system of Xray spot size for the 12MeV LIA is described in this paper. The Xray spot size and modulation transfer function, which judges the space resolution of 12MeV LIA Xray image, for the 12MeV LIA can be obtained using this system. According to the characteristic of approximate axial symmetry of Xray source and the distribution of image black density, the spa ce distribution of spot intensity is also imitated．