Abstract: The step calibration signal has been widely used in seismograph calibration and check. Routine check is currently being made with step calibration signal at Standard Chinese Seismic Stations. However this check can only show qualitatively whether the instrument working state is normal and does not determine quantitative variations in the elementary constants of a seismograph. Therefore it is not effective in monitoring variations in instrument constants and thus the significance of impulse check is reduced.Some foregoing studies deal with the quantitativ relation between impulse check and instrument elementary constants, However they only discussed separately the effect of variations in period, damping constant, and magnification respectively on the response waveform. But in a practical observatory the variations in instrument elementary constants are. interrelated.In this paper the equation of motion for checking instrument by using the step calibration signal is derived and solved by Laplace transform. An analytic expression for instrument response is obtained. Then some cases of instrument parameter variations encountered in practice and their relation to instrument elementary constants are discussed. The instrument response waveforms of the above cases are calculated. The concept of characteristic parameters of response waveform is introduced for analyzing the relation between response waveform with instrument parameters and the interrelations between variations of elementary constants. The minimum variation range of characteristic parameters that can be identified on seismograms is given. Thus quantitative relations between elementary constant variations and the characteristic parameter variations of response waveform are established.At the end of this paper the variation ranges of elementary constants identified possibly in various variation cases and their reliablity are given.