This paper proposes a method of determining design ground motion for critical engineering structures, based on Probabilistic Seismic Hazard Analysis (PSHA). Effective peak acceleration (EPA) is used as basic ground motion parameter in this paper. Comparing the contribution of dominant potential seismic source at long period (T=I.Os) and short period (T=O.Is), we suggest that the dominant potential seismic source be determined according to the dynamic property (e.g. expected period) of the engineering structures. And then we utilize the contribution functions of some dominant sources, and attenuation law to determine design earthquake magnitude and epicentral distance. According to these results, design ground motion parameters could be determined reasonably.