Model for description of evolution of the energetic spectrum of relativistic electrons downstream of the shock which accounts for electron radiative losses is presented. It may be used for calculations of X-ray synchrotron emission from supernova remnants in uniform interstellar medium and uniform interstellar magnetic field. The influence of different factors on spatial distributions of nonthermal electrons and their synchrotron emissivity is shown. The errors in estimations of magnetic field strength with models neglecting detailed picture of evolution of magnetic field and electron spectrum downstream of the shock are demonstrated. The evolution of sunchrotron spectrum and the ratio between radio and X-ray synchrotron fluxes are calculated.