Abstract: The seismic exploitation plays an important role on looking for possible hydrocarbon structure,coal geology structure.And the research is based on elastic wave equation of single-phase media,ignoring the liquid influencing on seismic wave transmit.This article discusses seismic wave field feature of free form gas on coal seismic data based on double-phase media theory,aimed at effectively judging abundance zone of gas,and controlling mischance arose by coal and gas outburst.