The structure of thick pinhole with double cones is studied and designed for the spot size measurement of the high-energy intense-current linear induction accelerator (LIA) light source. The theoretical model for numerical calculations is built up according to the parameters of the light source and the practical layout of the experimental measurement. The imaging process of photons penetrating through the thick pinhole structure is simulated, and the impact of the spot size, distribution and off axis on the source spot measurement is analyzed. Calculated results show that the spatial resolution on the source plane reaches 5 lp/mm.