The high intense pulse electron beam emitted and accelerated by linear induction accelerator (LIA) is focused to heavy metal target to produce X-ray pulses via bremsstrahlung mechanism. The X-ray is applied to high energy flash radiography. The X-ray spot size is a critical parameter for LIA and the main factor which degrades resolution of the flash radiography. This paper describes a pinhole imaging system measuring the X-ray spot size. The full width at half maximum (FWHM) of the X-ray spot size can be obtained from the pinhole image data. The modulation transfer function (MTF) which is derived from the X-ray spot image by Fourier transform is applied to calculate the 50%MTF spot size. In the continuous experiments of multi-pulse electron linear induction accelerator (MPELIA), the X-ray spot size is measured and the results demonstrate reliable performance of MPELIA. The concept of form factor is introduced, and the measured results show that the MPLIA X-ray spot distribution changes between Gaussian and Bennett distribution.