The first seismometer in the world was invented in 132 AD by Zhang Heng in the Eastern Han Dynasty and the birth of this instrument had strongly social and historical background. For Zhang Heng's scientific idea, the present authors would like to discuss its forming process and material basis. Zhang Heng lived in a highly developing period of culture and science in the Eastern Han Dynasty. The original pursue of studying and designing such instrument in ancientness was just a quite simple desire to observe nature anomalies for future prediction. The authors in this paper review briefly the social background at that time, the academic course of Zhang Heng and the historical process of the instrument creation. The paper also analyses the seismic activities in early Eastern Han Dynasty, ancient ideas for earthquake, as well as scientific activities carried in the ancient observatory ( Ling Tai). It appears that corresponding study work of the instrument started in about 128 AD. Based on his experience, Zhang Heng came to two simple and unsophisticated concepts of earthquake, i. e. the quake position could be judged according to its orientation and the earthquake should be distinguished seriously from landslip. Such perceptual knowledge was very significant and made the innovation following clearly after technical requirements. The general principle of Zhang Heng's making is to imitate the objective world as much as possible. The principle of his seismoscope includes the imitated response of hanging object to earthquake, the detecting technique of triggering bolter mechanism, and the recording system realized by dragons spitting a ball. Successful settlements of above three key problems in science and technique made Zhang Heng's innovation completed. The birth of Zhang Heng's seismoscope indicated the first successful use of material inertia by mankind, and had positive social effects in history.