Abstract: Small earthquakes used as the standard of section seismic checking in seismic design confuses two significantly different concepts. One is the ground motion parameters corresponding to 63% exceeding probability in 50 years (hereinafter referred to as frequent earthquakes), and the other is the parameters received by reducing that corresponding to 10% exceeding probability in 50 years (hereinafter referred to as reduction small earthquakes). Above confusion leads to a result that two different standards are actually followed in seismic design. Complexity of the relation of ground motion parameters corresponding to medium and small earthquakes is discussed, such as dispersion of peak acceleration, differences of consistent probability spectra on the bedrock obtained from seismic hazard analysis, and systematic differences of site-related response spectra between medium and small earthquakes by soil non-linear effect. Substitution frequent earthquakes for reduction small earthquakes in section seismic checking for major engineering projects may result in underestimation of response spectrum characteristic period Tg. Some suggestions are put forward.