The invention relates to a method of mensurating steel residual stress using X rays, comprising: (1) taking a galvanized sheet, polishing smooth surface layers; taking saturated NaCl aqueous solution as electrolyte, using measured components as anode corrosion ends, using galvanized sheet iron as a cathode, eroding by DC constant pressure, corrosion current being 7-8 A; (2) after corrosion cleavage, mensurating residual stress on vertical direction of welding using X rays, then correcting residual stress measurement value after cleavage using single side cleavage measurement stress value correction method, namely obtaining steel residual stress. According to the inventive method to measurate the steel residual stress, novel additional stress is not increased by electrolytic corrosion cleavage, also measurement result along heavy gauge workpieces is obtained, measurement result is simple, device cost is low.