Abstract: The characteristics of explosive seismic source has been investigated with close-in ground motion data. On the assumption of half space model, the underground explosion seismic source function, i.e. reduced velocity potential (RVP), has been obtained deducting the ground surface response. The spectrum magnitude of RVP and its parameters including steady state value , frequency factor and overshoot B were obtained. The relationship between yield and source parameters of and were obtained as well.