Abstract: This paper first shows some results obtained by rock mechanics experiments,source physics research,and earthquake prediction. The common problems involved in those researches need to be solved by geohysical experiments of medium size (of km size). It also gives a summary about the geophysical experiment field of km size in Mentougou,Beijing. Finally,Some results obtained up to the present are given,such as the differences between records on the surface and underground; the relation between stress drop of microfractures and stress state; the relation between the predominant frequence of background noise and stress state of the media; as well as characteristics of records recorded in the source region.