The equation of X-ray flux was deduced with the transverse divergence of electron and laser beams in the interaction area being taken into account. The relationship between the size of electron and laser beams and both X-ray flux and spectra flux was analyzed. The performance of the proposed LSS experimental facility based on the ps

fs electron accelerator of SINAP was calculated with the electron and laser beam sizes at the interacting point being determined by the criteria of maximum spectra flux.