This paper collected the data of gravity changes of high precision gravity resurveys networks and earth tide stations during 1970-1985 in China. The relationship between the gravity changes and 11 earthquakes M≥5.0 around the 6 routes and 5 stations has been analyzed. And then the possible mechanisms and characteristics of gravity changes with time have discussed. It is believed that if the moderate strong earthquakes will occur within certain distance from the station, then gravity changes of proper measure would appear before and after earthquakes. The duration T or Tc of gravity change is dependent on the magnitude M, the epicentral distance L and the area of aftershocks S or volume of source V. The gravity changes accompanying an earthquake are mainly caused by underground mass density change apart from crustal deformation. Different features of earthquakes may be verified by the data of gravity and deformation.