A detailed numerical simulation study of spectral characteristics of the x-ray flux from gold foil targets of different thickness irradiated by a 1ns laser pulse with 1014W/cm2 intensity at 0.35μm wavelength using the one dimensional radiation hydrodynamic code MULTI is investigated using one dimensional radiation hydrodynamic code MULTI.This is very important in the diagnosis of the x-ray spectrum and the design of an x-ray source.